There is nothing like defeat as a catalyst for change in the life of political parties. Stung by the loss of Congress and dealing with a president in a tailspin, the Republican Party is now engaged in a bruising battle to loosen its longstanding ties to social conservatism and reshape its image for mainstream voters.

The party’s 10 White House hopefuls are at the forefront of this intraparty struggle, which is intensifying with each of their nationally televised debates. Their most recent, in South Carolina last week, erupted into fits of name-calling and flashes of anger as the leading candidates’ conservative credentials were questioned by those who consider themselves the truer conservatives.

To see the possibility of a dramatically changed GOP, look no further than the men who, by dint of opinion polls and their own organizational and fundraising strength, are in the current 2008 presidential field’s top tier: Sen. John McCain of Arizona, former Mayor Rudolph Giuliani of New York and former Gov. Mitt Romney of Massachusetts. It is odd for a party so bound to its traditions that its leading candidates are the ones challenging the party’s past — although in different ways and to varying degrees.

Whether by virtue of their record on hot-button social issues (abortion, guns and gays) or on the strength of their current stands, the Big Three — even when seeking to distance themselves from their moderate-to-liberal pasts — are very much at odds with the others, all of whom are eager to deride their rivals as faux conservatives.

Watching Rudy McRomney (as one of the others, former Gov. Jim Gilmore of Virginia, dubbed the group) field those attacks has become great sport. What remains remarkable is the sight of moderates at the top of the heap instead of on the sidelines, where their kind has sat in Republican primaries for the past three decades.

Even more amazing is that the presumed front-runner, Giuliani, is the one who most clearly represents a fundamental shift away from social conservatism. He tries not to emphasize that he’s out of sync with so many primary voters, but he’s also not trying to pretend to be something else. Instead, he tries to change the subject. At last week’s debate, he managed a clever dodge to a direct question about his social values. Asked if his “pro-choice, pro-gay rights, pro-gun control” views are the “stands of a conservative,” Giuliani offered no pretense of answering the social-values question — and instead went after one of the Democratic front-runners, Sen. Hillary Rodham Clinton. His fellow New Yorker opposes a free-market economy, Giuliani declared, and believes government has a right “to take money from you in order to give it to the common good.”

By presenting himself at that moment as an economic conservative who can take it to the liberals, Giuliani was essentially asking GOP primary voters to make that more important than his social stands. And he is pressing the argument that his ideological mixture is more marketable in a general election. While McCain and Romney are more equivocal, they also represent a new direction for Republicans. McCain still suffers fallout for chastising evangelical Christian leaders during his failed 2000 campaign. Romney is trying to run away from his liberal record on social issues as a governor and former Senate candidate in his “deep blue” home state, but consequently he’s getting tagged as a flip-flopper.

Unless an unambiguous social conservative emerges to seize the nomination, it now seems destined to be won by one of the Big Three. And at that point, the winner will probably yield to pressure for a genuinely conservative vice-presidential nominee.

Enter most of the rest of the current crop of GOP contenders. In an odd way, the social conservatives in the race can best earn running-mate honors by jabbing at the leaders, thus rallying core voters to their side. That sure seemed to be at work in last week’s debate. One to watch is former Gov. Mike Huckabee of Arkansas, a Baptist preacher who is impressing Republican operatives and media commentators with a personable, upbeat style that gives no ground on his firmly conservative social views. Huckabee is gaining appeal among conservatives despite complaints that he backed too many tax increases as governor.

Still, the mere possibility of a socially liberal nominee such as Giuliani — even if he is forced to pick his mirror opposite as a running mate — would be nothing short of a sea change for the party. While Giuliani’s maverick views might represent a rising GOP tide that would seriously threaten the Democrats, there is a showdown coming. Either someone in the current crop of social conservatives will effectively challenge him, or a newcomer to the race, such as former Sen. Fred Thompson of Tennessee or former House Speaker Newt Gingrich, will jump in to claim the mantle.

How this fascinating struggle ends will probably determine the Republican Party’s fate for years to come.

Contributing Editor Craig Crawford is a news analyst for NBC, MSNBC and CNBC. His new book is “The Politics of Life: 25 Rules for Survival in a Brutal and Manipulative World.” He can be reached at ccrawford@cq.com.