Marco Rubio's Biggest Problem Isn't Jeb Bush

The friends-turned-rivals narrative is salacious, but the numbers show that Rubio should be worried about a different candidate.

At first glance, Jeb Bush looks like the biggest obstacle to Marco Rubio's presidential aspirations. Both hail from Florida, both are touting a reform-oriented brand conservatism, and it makes for good political theatre: The two have been friendly since Rubio's tenure as speaker of the Florida House. In search of a distinction against his one-time mentor, Rubio emphasized that he was the candidate of the future in his kickoff speech. It was a subtle nod to the fact that there's not much else that separates the two.

But if Rubio is ever going to get a chance to face off with Bush, he has a more pressing problem to deal with first: a brewing collision with Wisconsin Gov. Scott Walker.

Despite Rubio and Bush's surface similarities, their bases of support are surprisingly distinct. Bush remains the clear favorite among Republican centrists, holding onto a slim plurality of support in most national polls thanks to his establishment connections. What's forgotten about Rubio, especially since his ill-fated immigration reform advocacy, is that he's still the favorite of many tea-party conservatives.

(RELATED: Inside Jeb Bush's Stealth Campaign to Woo Christian Conservatives)

This week's CNN/ORC national survey of GOP voters found Rubio leading Bush, 14 percent to 11 percent, among Republicans who identify with the tea party. But the same poll showed Rubio trailing 20 percent to 7 percent among those who don't. In their home state of Florida, this month's Quinnipiac survey showed Rubio with an 81 percent favorability rating with tea-party supporters, while Bush's was only 57 percent.

Rubio and Walker, on the other hand, are locked in a zero-sum game for the same pool of votes. Both have strong support from the activist wing of the Republican Party while being acceptable to the party's establishment. In recent Quinnipiac polling of the Iowa caucus, my colleague Scott Bland found that Rubio and Walker's support overlaps the most. (Rubio and Bush supporters are among the least compatible in the entire field.) Like Rubio, the poll shows Walker faring better with conservatives than with centrists. It's no coincidence that Rush Limbaugh championed both of their campaigns on his radio show—they are young, accomplished conservative leaders who share a track record of winning tough elections.

But while they are the most viable conservative alternatives to Bush, they espouse different visions for the party. At a time when GOP voters are attuned to electability, a choice between Rubio and Walker would require choosing between two paths to the presidency.

(RELATED: The Great Risk in Marco Rubio's 2016 Run)

Nominating Walker would be a bet on the party's base of white, working-class voters, hoping to translate his political accomplishment of mobilizing core conservative voters to a national stage. Walker's message of governing as a principled conservative in a blue state, picking and winning fights with the Left, is one that's designed to run up the score with the party's core constituents, but holds less appeal to the diversifying electorate that Democrats are betting on. The Walker model would be a play for flipping several of the Midwestern "blue wall" states that have voted Democratic in every presidential election since 1992—including Pennsylvania and his home state of Wisconsin.

By contrast, Rubio's message is designed to imagine how the party frames conservatism for a future generation. His reference against the politicians of yesterday in his kickoff speech was a dig at Clinton (and more subtly, at Jeb) but it also should be seen as a criticism of his own party's direction. His signature economic proposal has stirred unrest within conservative circles because it breaks from the GOP principles of cutting marginal tax rates in exchange for family-friendly tax credits.

And by nominating the first Hispanic presidential candidate and appealing to a younger generation of voters, Republicans would be able to make a case for broadening the party's appeal beyond its core of older, whiter voters. If Rubio is able to make even small inroads with Hispanic and millennial voters, it would make Hillary Clinton's path to the presidency much more challenging. He'd be better-positioned to lock down the must-win state of Florida, and he would be a better fit for winning back perennial battleground states Colorado and Virginia.

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Education level will be a big dividing line between Rubio and Walker. There's a sizable gap in Rubio's support between college-educated conservatives and those who didn't attend college, according to the CNN/ORC poll. With 15 percent support in the crowded field, Rubio places a close second to Bush among those who attended college, but languishes at five percent among non-college Republicans. Walker's support is more balanced, with 12 percent support among college-educated and nine percent among non-college.

If electability plays a larger role in voters' priorities, expect Rubio to hold an advantage in pressing his case. Walker's argument of being able to expand the map is being undermined by his sagging approval ratings back home. A Marquette Law poll of Wisconsin registered voters shows his disapproval rating at an all-time high—56 percent—and shows him losing to Clinton in his home state by 12 points, 52 to 40 percent. Rubio's overall favorability ratings back home are in better shape, with 42 percent positive and 38 percent negative in a late-March Quinnipiac survey. In the Quinnipiac survey, he's within two points of Clinton in Florida, and leads her by six points in a Mason-Dixon poll out this week.

Assuming Rubio can translate his political talent into early state success, his clash with Jeb Bush will come towards the later stages of the nominating fight—perhaps with the winner-take-all March 15 Florida primary. But to get there, he first needs to prove he's the most electable conservative. National polls have shown that Rubio has gotten momentum since his kickoff announcement. But to maintain that momentum, he'll have to disqualify Walker in the process. The battle between the party's two rising stars for the opportunity to take on Bush will be heating up sooner than later.