Elizabeth Warren today told Fortune magazine that she won’t run for president. If Warren stands by that decision, she’ll do a tremendous disservice to her principles and her party.

Warren is the only person standing between the Democrats and an uncontested Hillary Clinton nomination. She has already made clear what she thinks of the Clintons.

Warren has suggested that President Bill Clinton’s administration served the same “trickle down” economics as its Republicans and predecessors.

Warren has denounced the Clinton administration's senior economic appointees as servitors of the big banks.

Warren has blasted Bill Clinton’s 1996 claim that the era of big government is over and his repeal of Glass-Steagall and other financial regulations.

Warren has characterized Hillary Clinton herself as a conscienceless politician who betrayed her professed principles for campaign donations.

When Warren said these things, political observers insisted that she was merely exercising her vocal chords, raising a flag indifferent to whether anybody saluted. Jill Lawrence expressed this view concisely in a Politico profile last week: "[Warren] remains vastly influential as long as she retains her unique role in the national conversation. But if she actually were to run, all that would change.”

But really: What kind of a role does a junior senator from the minority party actually play? Yes, she just stopped a third-tier Treasury nomination. Congratulations. She can’t stop two. She certainly can’t enact laws. She cannot (as she painfully discovered during the December cromnibus debate) stop Republicans from unraveling Dodd-Frank stitch by stitch. Once the presidential contest begins in earnest, she’ll be pressured to join the cheering squad for the achievements of the Larry Summers-Bob Rubin years—and to keep silent as Hillary Clinton raises hundreds of millions of dollars from Wall Street Democrats.

And if Hillary Clinton wins in 2016, what role for Warren then? President Clinton will face Republican majorities in both House and Senate. Like her husband in the 1990s, she’ll have to do business with them—and squash any Democrat who objects. If Hillary Clinton loses in 2016, Warren’s role in the Senate will quickly be eclipsed by the next generation of Democrats competing for their chance in 2020. By then, Warren will be nearly 70, older than most presidential candidates, even in our geriatric political era.

On the other hand, you know who plays a truly significant role in the national conversation? First-tier candidates for president, that’s who. Hillary Clinton understood that truth when she ran for president as a not very senior senator in 2008. Yet when Hillary ran that first time, she didn’t have much to contribute to the conversation. She ran, as the saying goes, because she wanted to be something, not because she wanted to do something. Warren plainly does want to do things—and is denying herself her best chance to get them done.

If Elizabeth Warren did seek the Democratic presidential nomination, she’d seize the party and the national agenda. Rank-and-file Democrats seethe with concern about stagnant wages, income inequality, and the malefactions of great wealth.

Left to her own devices, Hillary Clinton will talk about none of that. Hillary Clinton is a candidate so cautious that, compared to her, Michael Dukakis seems the second coming of William Jennings Bryan. Everything about her is polled, focus-grouped, and second-guessed. Her policy positions are measured in millimeters to the left of center. Her speeches are written first and foremost to ensure they can never be quoted against her. How many people remember what Hillary Clinton accomplished as a U.S. Senator? As a secretary of state? Since the fiasco of her 1993 healthcare initiative, Hillary Clinton has so feared doing the wrong thing that she has almost always opted to do nothing.

Lead a fight for America’s working people? Hillary Clinton wouldn’t lead a fight for motherhood and apple pie if motherhood and apple pie were polling below 70 percent.

Nor would it likely assuage Elizabeth Warren if Hillary Clinton ever did speak or act boldly on behalf of Clinton’s core convictions. Few presidential candidates since William McKinley have had more personal, financial, and political connections to America’s wealthiest people than Hillary Clinton. She and her husband have gained a fortune estimated at $100 million that was, to put it bluntly, more or less donated to them by their friends and supporters. Hillary Clinton may gravitate to the less reactionary and more public-spirited billionaires. Yet it’s still billionaires, billionaires, billionaires all around her.

Worst of all, from Warren’s point of view, would be the policy ambition of a Hillary Clinton presidency. The big ideas exciting the Democratic center are free community-college education (advanced by President Bill Clinton back in the 1990s, now endorsed by President Barack Obama) and some kind of national pre-kindergarten program. Both initiatives are premised on the assumption that wage stagnation is traceable to educational deficiencies. These initiatives would be very expensive, but ultimately they are not very radical: They seek to improve the American worker, not to reform the American job market.

What if you agree with the Democratic left that the problem is not the employee, but the employer? What if you think that Thomas Piketty is right, that capital is overpaid, that the wealthy have gained too much political power and are using that power to enrich themselves further? You’re not likely to get much from a Clinton presidential campaign, still less from a Clinton presidency.

Only one thing could change this dreary calculus: a credible challenge from Hillary Clinton’s left. Such a challenge would force Clinton to shift left—and might extract commitments that would bind a future Clinton presidency, as the right extracted commitments from Mitt Romney in 2012. Even better, from the left-wing point of view: A left-wing challenger might actually win.

Just why, after all, is Hillary Clinton so inevitable? What does Clinton bring to this contest that she didn’t bring to the contest she lost in 2008? Have her ideas become more exciting? Her speeches more inspiring? Her story more relatable? The only difference this time is that there’s no alternative candidate competing against her. And that difference could change with one word from Warren.

Could Warren do it? Of course she could. More than almost anybody running in 2016—more even than Republican insurgents like Ted Cruz and Rand Paul—Warren has both her message and her constituency ready to hand. Hillary Clinton speaks to those Democrats who feel that Barack Obama went too far. Elizabeth Warren speaks to those Democrats who feel he didn’t go far enough. And if Warren’s supporters aren’t as spectacularly wealthy as Clinton’s, together—as Barack Obama proved in 2008—they can give more than enough to fund a winning campaign.

What about the general election? Not since 1960 has the Democratic party won the presidency with a Massachusetts liberal, and even that victory proved a squeaker. But elections are comparisons, and if Warren has weaknesses in such a contest, Hillary Clinton has more. Suppose the Republicans nominate Jeb Bush, as seems at least plausible. What’s the Clinton message in such a contest? “My husband had a better job creation record than your brother”? She won’t be able to portray him as a candidate who owes everything to his famous last name. She won’t be able to ask questions about how he made so much money so fast without delivering any real world good or service to anybody. She won’t be able to dismiss him as out-of-touch with the realities of everyday life. She can’t say that he’s a throwback to the politics of 20 years ago. Each and every one of those most promising lines of attack on Jeb Bush will be foreclosed to Hillary Clinton, because every one of them will be even more damaging to her than to him. But Elizabeth Warren can speak to them. There’s no national Democrat who can draw a sharper contrast with Jeb Bush than Warren; no Democrat who has more in common with him than Hillary Clinton.

By now Warren knows (assuming she didn’t know before she arrived there) that the only thing the Senate can offer somebody like her is the velvety asphyxiation of every idealistic hope. If what you like best is the sound of your own voice and the deference of those around you, then a senatorship is a wonderful job. If you’re in politics to accomplish things, the institution must be almost unbearable. Can Warren bear it? The endless talk, talk, talk? The scoldings from White House aides whenever she says or does something they deem unhelpful? The merciless editing of her speech at the next Democratic National Convention —and the surgical exclusion from the innermost council of the party leadership? That’s the “unique role in the national conversation” in which a Hillary Clinton led Democratic party will cast Elizabeth Warren. Warren's got nothing to gain from staying put in the Senate except drudgery, ineffectuality, and humiliation.

If a politician expresses ideas that are shared by literally tens of millions of people—and that are being expressed by no other first-tier political figure—she owes it to her supporters to take their cause to the open hearing and fair trial of the nation. It would be negligent and irresponsible not to do so. Elizabeth Warren belongs to that unusual group who stick by their principles even when it might cost them something, including an election. But if you’re willing to lose for your principles, surely you should be willing to try to win for them?