But the shattering of that consensus isn't revealing new divisions within the party as much as laying bare old ones. Dating back to Sen. Henry Cabot Lodge (R-Mass.) -- who took on Democratic President Wilson over the League of Nations and won -- an isolationist strain has always run through the party, even if it remained on the fringes. The more prominent battle within the GOP, however, has been between Republicans arguing for aggressive foreign intervention, as President Reagan did, or those wary of overseas involvement and promoting a more pragmatic view, like President Nixon.
In the decade following 9/11, an aggressive foreign policy led by neoconservatives and supported by President George W. Bush and McCain controlled the party. But amid a rising tide of public opposition and soaring deficits, those conservatives derided by critics like McCain as "isolationists" have regained their footing in the party's debate.
"I think the candidates are reflecting a division among Republicans, many of whom, because of their concern about the current state of economy, have decided we can't really do both: engage in the rest of the world and fix our economic problems," said former Rep. Jim Kolbe (R-Ariz.) the former chairman for House Foreign Operations Appropriations Subcommittee. "So they are taking a more pronounced view of that, which would border on isolationism or withdrawal or retreating from our international stage."
Public opinion is also forcing the party to change course: The general public overwhelmingly, 59 percent, said they want the troops home as soon as possible in a mid-June poll from the Pew Research Center.
And 43 percent of Republicans want to quickly withdraw from Afghanistan, the Pew survey reported, a nine-point jump from a month ago. And 42 percent of tea party supporters favor withdrawal.
Support for overseas conflicts hasn't completely reversed itself in the GOP, however. An early June poll from CNN/Opinion Research found 70 percent of Republicans said it was necessary to keep U.S. troops in Afghanistan to prevent more terrorist attacks. The mixed results are one reason why Republican candidates find themselves on different sides of the issue -- or, in the case of early front-runner Mitt Romney, they appear to be trying to have it both ways.
Massachusetts' former chief executive has argued both in favor of removing troops quickly and listening to military generals who want to keep them there longer than the president.
"It's time for us to bring our troops home as soon as we possibly can, consistent with the word that comes to our generals that we can hand the country over to the Afghan military in a way that they're able to defend themselves," Romney said during his first GOP debate earlier this month.
The political impact of the differing foreign-policy positions could be stark: Not because war is a key issue in this year's election, but because it's one of the few areas where the candidates have substantive differences. The candidates mostly all espouse the same position on social issues, and although their economic policies differ in some cases, each candidate vigilantly opposes new taxes and supports deep spending cuts. Huntsman, Rep. Michele Bachmann, Romney, and Pawlenty have each also at least partially embraced Rep. Paul Ryan's proposal to overhaul Medicare.