McCain, a key negotiator on the compromise, seemed to be keeping his distance when the deal was announced last week. Now he has decided to tackle the matter head-on, frustrated by what his aides called pandering by his rivals and buoyed by polls showing that a majority of Americans supported a welcoming approach to immigrants.
He directly challenged one leading critic, conservative talk show host Sean Hannity, who warned Thursday of a growing anger among conservatives and a "groundswell of opposition" in the GOP base.
"So I am supposed to gauge my behavior on whether I am booed or not? Please, Sean," McCain responded during an appearance on the show.
By embracing immigration in Miami, McCain will be staking a claim to a key issue in an early primary state that Giuliani and Romney have made central to their strategies for winning the nomination. Romney and Giuliani have both hired aides to former Florida Gov. Jeb Bush. But the president's younger brother, whose wife is Mexican American, has expressed dismay about the anti-immigration views held by some in his party.
Aides say McCain, like the president, understands the importance of building ties with Latinos.
"We're getting close to the point where we will no longer be a national party if we try to define it as a white male, cul-de-sac, gated-community party," said John Weaver, McCain's chief strategist.
Some strategists think the GOP field is being pulled to the right in part by Rep. Tom Tancredo (R-Colo.), who is not a top-tier candidate but is running as the most prominent anti-illegal immigration member of Congress.
In an interview, Tancredo accused Bush's allies of risking the party's future by ignoring the GOP base. Deflecting criticism that the immigration issue hurt Republican candidates last year, he argued that the party alienated Latinos in 2006 for the same reason it lost support of other voters: the Iraq war, scandals and other administration failings.
"We lost a lot of white males too," he said.
The grass-roots anger at the party elite was on display last week in Georgia and South Carolina, when Sens. Saxby Chambliss (R-Ga.) and Lindsey Graham (R-S.C.) were heckled by otherwise friendly Republican audiences for their support of the immigration measure. Graham was booed when he mentioned that Bush understood the politics of the issue and performed well among Latinos.
Nationally, exit polls show the GOP share of the Latino vote dropped sharply from an unusually high 40% in 2004 -- the result of intensive outreach by Bush's campaign -- to 30% in 2006. Democrats see an opportunity to expand their share of the Latino vote, an important bloc in states such as Florida, New Mexico, Nevada, Arizona and Colorado that are expected to be pivotal in the 2008 presidential race.
Surveys compiled by the pro-Democratic group NDN, and not disputed by leading Republican strategists, show that immigration has rocketed from near the bottom to near the top of the list of concerns to Latino voters. More than half, the NDN surveys show, say the issue increases their interest in voting.
Some Republicans in Florida, Texas and Arizona performed well among Latino voters last year. But analysts say the national trend and the tenor of the current debate could spur the kind of realignment that boosted California Democrats after Republican Gov. Pete Wilson's 1994 embrace of anti-illegal immigration measures.
"Republicans have become a more menacing party to Hispanics over the past year," said Simon Rosenberg, president of the NDN, which has spent millions of dollars targeting Latino voters and documenting the pre-2006 GOP gains.
The tone of the 2006 campaign, along with recent comments by Romney and others, has at least one lifelong Republican questioning his loyalty.
Lionel Sosa worked as a political strategist for Bush, President Reagan and President George H.W. Bush. But if the eventual Republican nominee adopts a harsh tone on immigration, Sosa said he would not vote for the candidate.
"Blood runs thicker than politics," said Sosa, of San Antonio, who is helping organize a fundraiser for New Mexico Gov. Bill Richardson, a Democrat who is Latino. "I'm not saying I would vote for a Democrat. But I'm saying I would not vote for a Republican who opposed immigration reform."