Liberal doves are feeling a deep sense of betrayal after watching their champion of peace drop bombs on an Arab country. If the war drags on inconclusively, or if Obama feels compelled to expand our involvement, their discontent will grow.
Then what? Then he could face what Lyndon Johnson faced in 1968: a Democratic primary challenger appealing to those tired of war and mistrustful of their president.
Whom might that be? Maybe former Sen. Russ Feingold of Wisconsin, who opposed the Iraq war and provided the sole vote against the original Patriot Act. Maybe Sen. Jim Webb of Virginia, who is not running for re-election and has criticized the Libya attack. Or maybe someone else.
It isn't important whether the challenger is plausible as president. What's important is that alienated Democrats have some way to express their anger and disenchantment. No liberal insurgent is likely to beat Obama. But Obama can lose even if he wins.
Eugene McCarthy, after all, came in second to Johnson in the 1968 New Hampshire primary. Ted Kennedy couldn't unseat Jimmy Carter in 1980. Pat Buchanan lost every primary against George H.W. Bush in 1992. None of those presidents, however, got a second term in office.
A Democratic challenge could be fatal to Obama's re-election bid, for several reasons. It would highlight the ways in which he has failed liberals. It would make him look beleaguered and vulnerable. It would drain resources that could be used against Republicans. And the fight could embitter many Democrats, inducing them to stay home on Election Day.
In 1968, McCarthy's campaign posters said, "He stood up alone and something happened." It could happen again.