Most Republicans in Congress come from safely gerrymandered districts and risk little by supporting Ryan's budget. (The greater risk for many would be not supporting it.) But that isn't the case for the presidential hopefuls, whose appeal must extend beyond the conservative base if they are to have any hope of defeating President Obama. To them, Paul Ryan poses a problem.

Ordinarily, when the presidential primaries heat up, national candidates assume the role of party leaders and set the agenda. But since none commands much support, none has anything approaching Ryan's influence. This has created an unusual situation in which the presidential aspirants are essentially bystanders and Republican politics are being driven by governors and congressmen. As one adviser to a presidential candidate put it to National Journal's Ronald Brownstein in March, ''This is the tail wagging the dog.'' Brownstein suggested that some of the extreme positions being imposed on the GOP presidential field -- intense opposition to collective-bargaining rights for public-sector unions; support for hardline anti-immigration laws like Arizona's -- could harm the party's nominee in the general election since President Obama and the Democrats will likely exploit them.

In fact, the damage is already being done, except it's occurring within the Republican Party and the dividing factor is Ryan's budget, particularly its plan to privatize Medicare. The budget's attempt to overturn the individual mandate in the new health care law has been problematic for Mitt Romney and Newt Gingrich, who supported mandates before the Tea Party arose, when doing so was considered acceptable for Republicans. On Sunday, Gingrich reiterated his support for mandates and then dismissed Ryan's Medicare plan as ''radical'' and ''right-wing social engineering.'' Conservatives excoriated him. In Iowa, television cameras captured a humiliating exchange with a voter who denounced his attack on Ryan and urged him to ''get out before you make a bigger fool of yourself.'' On Tuesday, Gingrich called Ryan and apologized.

All this is testimony to Ryan's impact. If there were any questions about primacy within the party, they're settled now. Gingrich's submission established the Ryan budget as the one true benchmark for activist conservatives. But even Ryan appears to recognize its drawbacks. On Monday, he gave a speech in Chicago in which he sought to recast his designs in a softer light. He barely mentioned Medicare.

The problem for Republicans is that this budget likely represents a high-water mark, and an unattainable ideal. A number of swing-district Republicans have met with an angry backlash. Congressional leaders have signaled that they won't attempt to enact it. And while a budget deal is unlikely anytime soon, congressional Republicans can in theory compromise under the right set of circumstances. But that will be much harder for a presidential candidate -- harder still now that Gingrich has capitulated.

Joshua Green writes a weekly column for the Boston Globe.

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