But primary challenges can have nobler purposes. For my generation, the greatest was Eugene McCarthy’s 1968 New Hampshire run against President Lyndon Johnson — mainly to protest the war in Vietnam — which heavily influenced Johnson’s decision not to run for re-election. Technically, McCarthy lost: He got 42 percent of the popular vote against 49 percent for the president’s still unofficial write-in candidacy. However, that was enough to force a genuine national debate on the war and Johnson’s decision not to run again. Similar to Obama’s primary challenge to Rush, McCarthy won by losing, although he didn’t get the nomination.

The same tactic, if not nobility, applies to Republicans. In 1992, Pat Buchanan “won” the New Hampshire primary with 40 percent, compared with President George H.W. Bush’s 58 percent. With no real hope of winning his party’s nomination (unlike Gene McCarthy), Buchanan was nevertheless determined to drive Bush from office. His protest candidacy helped do that by weakening Bush before the general election.

Of course, primary challenges needn’t be reserved for epoch-making presidential elections. When liberal friends bitterly complained to me about Senator Patrick Leahy’s (D.-Vt.) 2005 Judiciary Committee vote to approve John Roberts’s nomination as chief justice, I suggested that they recruit a candidate in Vermont to jam up Leahy’s usually automatic re-election. Now that Roberts has steered the country to even greater domination by corporations in Citizens United v. Federal Elections Commission, why not mount a serious liberal primary challenge to protest Leahy’s terrible decision? Even if Vermonters renominate him next Sept. 14, a chastened Leahy might think twice during, say, a Sarah Palin administration, about supporting another right-wing Supreme Court nominee. He might even filibuster!

So who is the best person to take on Obama? My first choice would be a rejuvenated Howard Dean, who might be the only hope left for the cause of liberal reform, at least in my lifetime. After scaring the wits out of the party’s Iraq-compromised establishment in 2004 with his anti-war and pro-small-donation crusade, Dean performed the thankless task of chairing the Democratic National Committee in some of its dark years. In that post, he compromised himself again and again in the interest of party unity and beating the Republicans in 2008. This involved the deeply unpleasant task of collaboration with the violent-tempered Emanuel, as well as with New York Senator Charles Schumer (another anti-reform Democrat), who was chairman of the party’s Senatorial Campaign Committee.

Yet for all his hard work, Dean was rewarded by being passed over for the new Cabinet and was not invited to continue running the DNC. What’s more, in an ugly bit of symbolism, RahmObama named as press secretary Robert Gibbs, who was spokesman for the Democratic 527 committee whose sleazy advertising contributed to killing off Dean in the Iowa caucuses six years ago.

Already detested by Emanuel (our new Dick Cheney), Dean has more recently committed the cardinal sin of party disloyalty by openly denouncing Obama’s (and Max Baucus’s and Joe Lieberman’s) attempt to transfer huge amounts of taxpayer money to Wellpoint and other big insurance companies. So what’s he got to lose, except maybe an election? The country, on the other hand, might have much to gain. And Obama could relearn an important lesson about politics, and the consequences of betrayal.