BEFORE anything else is said, they deserve the thanks of a grateful nation. Bernie Sanders, Martin O’Malley, even Lincoln Chafee: They are stepping up where others quailed, laying their bodies on democracy’s altar, saving their party’s nominating contest from resembling a presidential re-election in Kazakhstan.

But if they aspire to more than just holding Hillary Clinton below the 97.7 percent of the vote that Nursultan Nazarbayev claimed in his last trip to the hustings, her primary rivals will need more than courage. They’ll need a plan.

A little while ago, the plan for a not-Hillary candidate looked obvious: While Clinton played it safe and hugged the political center, her challenger would run hard to her left, channel the energy of the party’s grass-roots activists, campaign against the front-runner’s establishment instincts and her husband’s triangulating past.

This road map did not promise victory. (Only Hillary herself can stop Hillary from winning — and even then not without a lot of work.) But it promised, at least, a meaningful battle of ideas, and maybe even a chance to make things close in Iowa.

Since her official entrance into the race, however, Hillary has moved aggressively to shrink the space for that kind of battle. Her big policy statements — on criminal justice reform, immigration, and now universal voter registration — have all aligned her explicitly with the party’s activists, and to an extent many of them did not expect.

Her theory seems to be that the political center has moved leftward, and that mobilizing Democratic constituencies will matter as much in the general election as reaching out to swing voters; she may also be haunted, understandably, by memories of 2008. (There’s no reason to think she isn’t sincere in her new stances, but with the Clintons it’s always fair to analyze strategy before belief.)

Whatever the motivation, this new positioning has made it even harder for the not-Hillarys to run against her. They can move further to her left (Sanders, in particular, won’t have trouble doing so), but then their campaigns will seem even more quixotic. And if Hillary stays committed to their major goals, the party’s activists will have clear incentives to just take her “yes” for an answer.

So what remains for our brave few, our band of brothers? Well, they can attack her as a latecomer, a flip-flopper, a fair-weather progressive. But such charges are rarely politically effective; if they were, Mitt Romney would never have been the 2012 G.O.P. nominee. At some level, voters know that they’re voting for the platform more than for the candidate. And the left, in particular, has philosophical reasons to be comfortable with flip-flops: If you think the Arc of History is bending toward your ideas, then it’s actually a kind of vindication when a politician bends your way.

Alternatively, our anti-Hillarys can focus on foreign policy, where her Iraq War vote helped doom her seven years ago. Here they’ll have a stronger case, since she probably remains more hawkish (see her role in our Libyan war for evidence) than her party’s dovish base.

But Clinton can find shelter by associating herself with the current president: She’ll just say (as she’s already saying) that she’s exactly as hawkish as Obama, no less and no more. If he escalates against ISIS, she’ll support it; if not, she won’t. Ditto Putin, Assad, you name it. This won’t protect her left flank fully, but so long as she’s hugging the president she’ll lose more left-wing intellectuals than actual progressive voters.

So all that really remains for her would-be challengers is to attack her ethics. There, at last, the anti-Hillary argument becomes an easy one: From the Nixonian style of her State Department operation to the way her family fattened itself on global tribute during her recent public service, her rivals can point to sins and misdemeanors that would have already disqualified a lesser candidate.

But will many Democrats really want to hear that argument? The advantage of making an ideological case against Hillary is that progressives can accept it — yes, she’s not as liberal as we would like, we’re glad somebody’s pressing her, and maybe we’ll cast a protest vote for them — and still feel O.K. about her inevitability and about voting for her in the general election.

The ethical case, on the other hand, is more personal, discomfiting, and easily repurposed by Republicans. So any time Hillary’s rivals offer those kinds of critiques, their audience will hear intimations of G.O.P. attacks to come. And since she’s almost certainly going to be the nominee, Democratic voters may not be particularly grateful for the foretaste; they may, instead, dismiss the men offering it for giving aid and comfort to the enemy.

In which case those men will still deserve our gratitude. Because then, at least, whatever happens in a Clinton presidency, her supporters won’t be able to say that they weren’t warned.