Movement conservatives have pushed Romney to take extreme positions throughout the 2012 campaign; they won't stop once he becomes president. As Grover Norquist explained in a speech to the Conservative Political Action Committee, "We are not auditioning for a fearless leader. We don't need a president to tell us in what direction to go. We know what direction to go. We want the Ryan budget .... We just need a president to sign this stuff." If Romney doesn't do as he's told, he will be in the same predicament as Jimmy Carter, who entered office with Democrats in control of both houses of Congress and yet found himself unable to move a domestic agenda because of intraparty bickering.

Romney's Impossible Task

Now assume instead the best-case scenario -- that Romney is an affiliated president carrying forward a still-robust Reagan regime. In that case, a Romney presidency will face two major challenges: factionalism and war.

As a regime ages, divisions emerge within the governing coalition. Affiliated presidents must find ways to give each faction something it wants without alienating the others. The difficulty is that, as time passes, the factions become more self-absorbed, insistent, and radical. Pleasing all of them may prove impossible; in that case, affiliated presidents have to choose which parts of the coalition to ally themselves with, risking the defection of the rest. This is the choice faced by presidents John F. Kennedy and Lyndon Johnson, who ultimately tilted in favor of a civil-rights agenda in the 1960s, alienating what had previously been the Solid South.

Affiliated presidents also face enormous pressures -- or temptations, depending on how one looks at it -- to use military force to display strength, both to the outside world and, equally important, to their political base. War hawks helped push James Madison into the War of 1812. James K. Polk avoided a war with Great Britain but ended up taking his chances on a war with Mexico. William McKinley found it politically impossible to resist a war with Spain. Sometimes, the results are politically successful; but sometimes, as in the cases of Lyndon Johnson and George W. Bush, they are not.

Romney will face these problems early in his presidency. He will inherit the leadership of a party with commitments to (1) further increasing tax cuts -- especially for the wealthy; (2) reducing deficits; (3) shrinking the size of government; (4) increasing defense spending; and (5) promoting a muscular foreign policy unafraid to use military force to solve foreign-policy problems, for example, in Iran and Syria. At the same time, Romney has promised to "save" popular middle-class entitlement programs like Social Security and Medicare, and to replace Obamacare with reforms that keep its most popular elements but jettison the features that make it economically practical. To top it off, he faces a reckoning in January 2013, when the Bush tax cuts expire and a sequester of defense and social programs goes into effect. That combination of tax increases and spending cuts will help solve the deficit problem, but it risks pushing the economy into a new recession, and it is completely unacceptable to the tax cutters and defense hawks in his party.

Even in the best-case scenario, a Romney presidency will face two major challenges: factionalism and war.

It is very difficult to see how Romney can maintain all of the commitments he has made to the various factions of his party, no matter what he says on the campaign trail. For example, passing the Ryan budget, further reducing tax rates, and repealing Obamacare will exacerbate the deficit problem, not help to solve it. Romney will have to pick and choose among these commitments, and in choosing, he will likely alienate significant segments of his coalition. Moreover, he will face insistent pressures from defense hawks and neo-conservatives in his party to keep the war in Afghanistan going and to use American military force against other targets. (Iran is the most obvious possibility.)