When Richard Nixon ascended to the White House in 1969, he was the first president since John Adams to enter office with two terms of vice-presidential experience under his belt. In his eight years under Dwight Eisenhower, Nixon remade the office, bringing new stature and power to the post. Arguably, no one had become president with a better idea of what made a good vice president. And arguably, no one made a worse decision in picking a running mate.

So why is it that running-mate experience failed to translate into running-mate wisdom for Nixon? His secret White House tapes, archived and analyzed at the University of Virginia’s Miller Center, offer new insights into Nixon’s fraught relationship with his vice president, Spiro Agnew. But more than that, they capture a moment in the new executive vice presidency that shows just how much the office has changed—and that help better evaluate Donald Trump’s and Hillary Clinton’s running-mate decisions.

Any discussion of the vice presidency usually kicks off with one of the dozens of quips about the uselessness of the office. Agnew contributed to the genre, observing that the vice presidency was “that rare opportunity in politics for a man to move from a potential unknown to an actual unknown.” And Nixon, after laying out a stirring defense of Agnew for reporters in 1972, went on to undercut him with a quote from Gilded-Age humorist Finley Peter Dunn: “The presidency is the highest office given to the people. The vice presidency is the highest and the lowest. It isn’t a crime, exactly. You can’t be sent to jail for it, but it’s a kind of a disgrace.”

Conventional wisdom holds that if the vice presidency matters at all, it matters only in its gestation period: the campaign. And vice-presidential selection has traditionally been attuned to that. Whether a sop to factions that lost the race for the top spot or an attempt to balance the qualities of the presidential nominee, the vice-presidential choice has generally had more to do with winning than with governing.

That was certainly the case with Agnew. Nixon initially chose him in 1968 because, as a moderate governor from a border state, he had both supported the civil-rights movement and made several tough-on-crime speeches. After the election, however, Nixon lost interest. He was eager to dump Agnew from the ticket when he ran for reelection in 1972, but he couldn’t. By then the vice president, with his attacks on the press and the political elites, had become a darling of a different faction: conservatives.

Had Nixon retained the right’s support during his first term, this wouldn’t have been a problem. But his decision to open relations with Communist China, in addition to his support for a raft of liberal social and economic policies on the domestic front, soured his relationship with conservatives. By 1972, they were in open revolt, even running a protest candidate in the primaries, Ohio Representative John Ashbrook. Nixon needed Agnew—not to govern, but to campaign. So Agnew stayed.

Nixon’s desire to dump Agnew stemmed from his belief that Agnew was too ineffectual for the office he held. The irony is that, thanks to Nixon, the responsibilities of the vice presidency had grown considerably by the time Agnew entered the office. Constitutionally, the vice president’s role was legislative, not executive. To the extent that vice presidents did anything in the pre-World War II era, it happened over in the Congress, where the vice president served as the president of the Senate, casting tie-breaking votes and certifying electoral-college counts.

As presidents gained more control over running-mate selection, a transition that began with Franklin Roosevelt in 1940, the office began to transform. The personal selection of a running mate enabled presidents to view their next-in-line as a team member. This was first fully realized during the Eisenhower-Nixon years. As vice president, Nixon abandoned the Senate and took up a number of executive functions, serving on commissions, liaising with Congress, and acting as an administration surrogate at home and abroad.

It was this new executive vice presidency that Agnew was supposed to fill. But while Nixon was happy with the electoral work Agnew did—1968 was a squeaker of a race, and Nixon believed Agnew had helped him win it—when it came to governing, he was deeply disappointed in his vice president.

Take Agnew’s foreign travels, modeled after Nixon’s own extensive globetrotting as vice president. A 1971 trip to Africa turned into a debacle for Agnew. In an Oval Office conversation with Nixon, John Ehrlichman, and H.R. Haldeman, in which the three discussed how to dump Agnew from the 1972 ticket, the Africa trip loomed large. Agnew’s constant golfing was a particular point of contention. “You’ve got to make it appear that the trip’s for work. Not over there on a goddamn vacation,” Nixon fumed.

He compared Agnew’s trip with his own travels as vice president: “Jesus Christ, you know, when I went on these trips with my wife, we worked our butts off, and it made an impression … [Agnew] had far more of substance than I had. But our trips really had a better effect, because, by God, we were out there talking to the people, visiting hospitals, and going through plants.”

Nixon was so frustrated with Agnew’s mishandling of the Africa trip’s optics that he gave his perpetual enemy, the press, a pass. In a conversation with Secretary of State Bill Rogers, Nixon noted how good the press coverage of his own travels as vice president had been. “I never had any friends in the press, you know,” said Nixon. “… But on my trips, my God, I got a good press. It’s almost impossible not to get a good press on a trip, mainly because of this: The press sort of is with you because, you know, you’re America over there, and they want you to do well.”

Agnew, though, gave the press nothing to cover in Africa—golf outings seldom make for good copy—and so the coverage focused on more earthy fare.

Haldeman: Newsweek. They [laughing] climaxed the report by saying that one of the highlights of this trip was leaving Kenya or somewhere in Africa where he and his personal physician and his very attractive red-headed secretary came down from their hut to watch a pair of rhinoceroses copulating.

Ehrlichman: [laughs]

Nixon: Holy shit. Really?

Ehrlichman: Yeah [laughs]

Nixon: Must be quite a scene.

Haldeman: Just look at those [laughing] fucking rhinoceroses!

Ehrlichman: Rhinoceri. [laughter]

Nixon’s criticism of Agnew’s press relations extended well beyond the Africa trip, however. Nixon is often portrayed as viewing the media only as his enemy. But Nixon had a nuanced strategy for the press and was quite critical of Agnew’s more ham-fisted approach, as his conversation with Rogers shows:

Rogers: I think that’s it. You know, that preoccupation of fighting the press all the time. Geez, you can’t win. It’s a losing battle.

Nixon: [speaking over Rogers] The point is to fight them but don’t appear to be doing it.

Rogers: I know it.

Nixon: That’s [Agnew’s] problem. He just wears it on his sleeve.

But all of this was secondary to a larger issue with Agnew: He wasn’t very good at being an executive vice president. He was too buddy-buddy with the Secret Service, too undisciplined with his staff. And for Nixon, this was a huge problem. “Everybody else raises points about Agnew, raises them for reasons that are wrong,” he told Ehrlichman and Haldeman. “I mean, taking on the press is fine, doing his political chores are fine, alliteration’s fine. But goddamn it, [pounding the desk] if he can’t run a staff, he cannot be in this office.”

Agnew, he concluded, was a local politician in a job far too big for him, too concerned about loyalty and not enough with competency.

Tangles with the press, paranoia, poor staffing choices—these could describe the Nixon presidency as much as the Agnew vice presidency. Which is the key to understanding Nixon’s running-mate problem: He over-identified with Agnew. As disparaging as Nixon was of Agnew, as much as he believed Agnew fell short of the high bar Nixon had set in the 1950s, his tendency to over-identify with Agnew was at its most damaging when Nixon focused on their similarities rather than their differences.

In June 1973, Agnew scheduled a meeting with Nixon to ask for more responsibility. He was hoping to run for president in 1976, but in his first four years in office, he had failed to distinguish himself in any meaningful way. Nixon, however, was not in a generous mood when it came to Agnew. He put off the meeting for a few days, complaining to Al Haig, “He is so parochial and selfish … He just wants to come in and whine. And darn it, I’m just not going to have any darn whining right now.” Nixon was so wound up that he spent the first few minutes of their meeting talking in asides to Haig, who had joined them for the discussion.

So Nixon did not immediately catch on that Agnew was not just there to shore up his presidential bona fides. Agnew had something else on his mind, an investigation in Maryland that seemed to be gathering steam. Meanwhile, Nixon’s mind was consumed with the Watergate hearings, which weren’t going well. That is perhaps why, when Agnew steered the conversation toward Maryland, Nixon kept reframing Agnew’s tale of aggressive prosecutors and possible kickbacks in terms of Watergate: the criminalization of politics, the persecution of the Nixon administration, the problems of immunity and plea deals. And when Agnew drew an explicit parallel to Watergate, Nixon went all in:

Agnew: [What] I’m going to tell you about now is something entirely different, to show you the lynch-mob psychology that exists over there. And this thing they’re calling a “little Watergate.” And this is the most ridiculous thing I’ve ever seen.

Nixon: Well, a “little Watergate” ought to be very small. What—you look at Watergate and all that was involved. What was it? A crappy little thing. A crappy little thing. There’s nothing there—they didn’t get anything. It hurt us in the election. We would’ve got 3 or 4 percent more. What in the name of good God is this all about?

Having made the connection to Watergate, an event Nixon was desperate to downplay, Nixon shrunk Agnew’s legal troubles down to the tiniest of nuisances. He immediately packaged the two scandals together and dismissed them both. “Just say that it’s persecution,” he told Agnew, offering up the line he used for the Watergate scandal. “Political, partisan, persecution crap.”

Agnew did. And it worked no better for him than it did for Nixon. The secret recordings stop a month after Agnew’s revelation, but it’s clear the relationship between the president and vice president continued to deteriorate. After Agnew’s resignation in October 1973, the two men never spoke again.

Agnew became vice president at a time when the office was in transition, when it was beginning to accrue new powers—but also when those powers were as yet undefined by precedent. And Nixon, jealously guarding his own power and eager to keep his inner circle tight, had little faith in Agnew. He trusted him with no responsibility, and Agnew remained stunted and dissatisfied in the position.

That failed partnership illustrates the real key to choosing a running mate in the era of the executive vice presidency. Think of all the working partnerships that followed: Al Gore worked alongside Bill Clinton to streamline government services. Dick Cheney helped George W. Bush radically expand the executive branch. Joe Biden became one of Barack Obama’s most trusted advisers. These relationships were not always warm—Cheney and Bush grew estranged in their second term, and both men’s effectiveness suffered as a result—but they were partnerships nevertheless.

And that is the lesson of Agnew, one worth considering in evaluating Trump’s and Clinton’s choices. Political scientists have shown that running mates no longer swing elections. But because of their expanded role within the administration, their most important qualities are their ability to work as part of the executive team and to govern effectively. Will Trump’s ego allow him to enter into a true partnership with Mike Pence? Will Clinton choose someone based on electoral politics instead of on governing prowess?

For all the quips and bon mots about the worthlessness of the office, the executive vice presidency no longer allows running mates to be nonentities. They may not make or break a campaign, but they will make or break a presidency.