To quote the old Sesame Street tune, one of these things is not like the others.

And yet Mr. Levin, a 73-year-old workhorse who dependably waved his party’s flag when it was far less fashionable to do so than it is now, suddenly finds himself the target of a stinging ad campaign from MoveOn.org, taken to task on his home state’s airwaves for voting against the Reid-Feingold Amendment, which would have barred funding for the Iraq War after next March.

The MoveOn ads won’t matter a lick when Mr. Levin stands for a sixth term next year. His solid reputation gives him more job security than just about anyone in Michigan.

Still, is this any way to treat a friend? After all, Mr. Levin, unlike many of his fellow Democrats, stood with MoveOn in opposing the Iraq War from the very beginning, refusing to vote for its authorization in the fall of 2002. And when Reid-Feingold failed by a 67-29 tally on May 16, his was merely one of 20 Democratic votes against it—some of them cast by Senators with far flimsier anti-war credentials. But MoveOn is using its funds to pillory Mr. Levin, and not any of them.

It’s easy to call this an exercise in cluelessness, an act of spite that will alienate someone who has been MoveOn’s legislative ally 99 percent of the time, with no measurable political benefit.

But consider that some of the other “no” votes were from Democrats who lack Mr. Levin’s electoral invincibility, whether in a primary or general election. The Levin ad—coupled with the memory of MoveOn’s role in derailing Mr. Lieberman in his primary last year—could be a warning to them: If Mr. Levin isn’t sacred, neither are you.

That threat-making is a powerful illustration of the broad sense of betrayal on Iraq that the Democratic base feels just five months after their party formally took control of Congress.

Grass-roots Democrats swallowed hard last year when the party establishment’s strategic calculations—like running anti-abortion candidate Bob Casey for the U.S. Senate in Pennsylvania—clashed with their hearts. Reid-Feingold was precisely the kind of legislation that was supposed to be their reward.

What followed was instead a crushing disappointment. Reid-Feingold failed (along with a comparable House measure), and then the party broke down over a supplemental war-funding bill: After initially uniting in a push to tie funds to a September 2008 withdrawal date, Congressional Democrats relented in the face of a veto and a follow-up veto threat by President Bush, leaving the war fully funded—with only the most cosmetic strings attached—through this September.

The resulting impulse among war opponents to lash out at establishment Democrats like Carl Levin, in that context, is understandable. But it’s also misplaced.

The rage of Democratic activists is based on a belief that ending the war over President Bush’s objections is simply a matter of will. Faced with Mr. Bush’s veto, they insist, the Democratic Congress needed only to send the same withdrawal-timetable legislation back to him again and again until either he relented and signed it or funding for the troops dried up, thus ending the war.

Maybe this would have been the heroic strategy, but it ignores the degree to which the Congressional leadership’s hands are tied. There is consensus among House and Senate Democrats about the need to end the war, but they are far from being of one mind on how to achieve this. Plenty of them believe in the aggressive strategy that the party’s base is agitating for. But plenty of them—even some who have long opposed the war, like Mr. Levin and Representative David Obey of Wisconsin—are also adamant that they won’t end the war by cutting off troop funding.

And it’s not as if the Democrats have a ton of wiggle room. In the House, they mustered 221 votes for the funding bill that Mr. Bush vetoed—barely more than the 218 needed to pass a bill. In the Senate, with Mr. Lieberman siding against them and South Dakota’s Tim Johnson still sidelined, the Democrats are a caucus of 49 on Iraq.

That doesn’t mean Democrats are waving the white flag. The funding question will be revisited in September, and already a chunk of previously loyal Republicans—wising up to their increasingly bleak 2008 prospects—have sent unmistakable signals to the White House that they’ll finally be prepared to declare their independence then.

Mario Cuomo once commented that in politics, you campaign in poetry and govern in prose—which helps to explain why activists were more enamored of Congressional Democrats in 2006 than they are in 2007.

There are honest differences among Democrats over how to end the war. But too many on the left are forgetting that the Carl Levins of the world are on their side.