Republican divisions over immigration loomed large as the 10 GOP presidential candidates gathered in the early primary state of New Hampshire for their third debate.

Hours before the debate, Sen. John McCain, R-Ariz., tried to pre-empt criticism of legislation he helped craft for President Bush, telling an audience in Gilford, N.H.: "Do I think it's perfect? No. I would remind you the Democrats are in majority in both houses now and we have to deal with them to resolve this issue."

McCain stands alone in backing the bipartisan bill that would tighten security on the Mexican border while allowing some of the 12 million illegal immigrants a chance to stay in the United States and eventually become citizens.

He has faced criticism from viable rivals and even lesser-known opponents, with former Massachusetts Gov. Mitt Romney and Rep. Tom Tancredo, R-Colo., seizing the mantle of some conservatives who oppose the deal.

Tancredo said Tuesday he would start a petition drive and volunteer network to help voters campaign against senators who support the White House-backed immigration plan. Romney refers to it as the "McCain-Kennedy" bill, linking his opponent to Sen. Edward M. Kennedy, D-Mass., a liberal icon.

Also participating in the debate were former New York Mayor Rudy Giuliani, Kansas Sen. Sam Brownback, former Govs. Mike Huckabee of Arkansas, Jim Gilmore of Virginia and Tommy Thompson of Wisconsin, and Reps. Duncan Hunter of California and Ron Paul of Texas.

The debate was sponsored by CNN, WMUR-TV and the New Hampshire Union Leader. The host was Saint Anselm College, which also hosted a Democratic debate on Sunday.

An eleventh potential GOP candidate, former Sen. Fred Thompson of Tennessee, did not participate in the debate, but the FOX News Channel invited him for a solo interview afterward.

Romney has complained that a so-called Z visa included in the legislation would allow the 12 million illegal immigrants already in the country to remain indefinitely, letting them jump ahead of other foreigners seeking to emigrate legally. He has proposed making the visas temporary, forcing recipients to seek permanent immigrant status or leave the country.

Nonetheless, one of his top economic advisers, N. Gregory Mankiw, former chairman of Bush's Council of Economic Advisers, signed an op-ed piece in Tuesday's Dallas Morning News supporting the bill.

"This is the most far-reaching and thoughtful reform of our immigration system in four decades and one that will significantly enhance American competitiveness," wrote Mankiw and his co-signers, including Jack Kemp, the 1996 Republican vice presidential nominee. "The benefits of the bill far outweigh its shortcomings. We believe it offers the only realistic way forward, and urge conservatives — and all Americans — to embrace the promise it holds out."

The third debate underscored an accelerated campaign this election cycle.