Few issues resonate with the conservative base more than illegal immigration.
It's a topic that's made for strange bedfellows in Washington -- President Bush and Sen. Ted Kennedy, D-Mass., have lined up on the same side -- and caused stark fault lines to emerge within the Republican presidential field.
Standing on opposite poles of this divisive issue are Sen. John McCain, R-Ariz., and Rep. Tom Tancredo, R-Colo.
McCain has lent his name to the comprehensive immigration reform bill backed by Bush and Kennedy, arguing that any approach to solving the problems on the nation's borders must be paired with a system that allows undocumented immigrants to get on a path toward legalized status.
Tancredo, meanwhile, has developed a national reputation as a hard-line foe of illegal immigration, and is basing his candidacy almost entirely on the issue.
He touts aggressive new border security measures and decries as "amnesty" any plan that would allow undocumented immigrants to gain legalized status.
The immigration issue will be front and center for the new president, given Congress' inability to pass a major bill this year.
Congressional leaders say the looming presidential election -- and the complicated politics it brings -- will almost certainly force the issue to be punted beyond 2008, leaving it as a major problem for Bush's successor.
Between McCain and Tancredo lie a range of different sentiments on illegal immigration, though the center lies closer to Tancredo than McCain.
The Arizona senator is essentially alone among Republican presidential contenders in advocating for the comprehensive bill that would allow virtually all of the 12 million undocumented immigrants now in the United States to achieve citizenship if they pay fines and back taxes, learn English, avoid criminal troubles and maintain steady jobs.
For most of the rest of the field, positions on immigration track a roughly similar path to the right, reflecting widespread concern -- particularly among conservatives -- about the explosion in the number of undocumented immigrants in the United States.
Former New York Mayor Rudy Giuliani last year voiced support for key elements of the McCain-Kennedy bill, including a path to citizenship for undocumented immigrants.
But this year as a presidential candidate he's grown into a harsh critic of that approach, calling for a nationwide database that would identify all undocumented immigrants and tamperproof ID cards to guard against fraud.
Former Massachusetts Gov. Mitt Romney also had kind words for a previous version of the McCain-Kennedy approach.
But he has built his campaign around strong border enforcement and criticized the manner in which this year's bill would give virtually all undocumented immigrants at least temporary legal status.
Sen. Sam Brownback, R-Kan., was one of McCain's co-sponsors on last year's bill, but he wound up opposing this year's version.
Like Romney, he cited the fact that the latest measure would provide legalized status to more undocumented immigrants and said he feared the bill would have the same adverse impact as previous attempts at granting "amnesty" to those who entered the country illegally.
Among some of the lesser-known candidates, Rep. Duncan Hunter, R-Calif., is challenging Tancredo for the mantle of favorite candidate of immigration hard-liners.
He has long been a proponent of a border fence, both in his Southern California congressional district and across the country, and has blasted the Bush administration for what he has described as lax border enforcement.
McCain himself has acknowledged that his stance on immigration has probably cost him support among Republicans, including with all-important political donors.
But he has remained outspoken in his support for a comprehensive approach to the issue, despite initial signals that he would distance himself from the position during his run for the presidency.