New Hampshire activist Bruce Keough is signing up with Massachusetts Gov. Mitt Romney's burgeoning presidential campaign for one reason: He considers Romney the opposite of John McCain.

The Arizona senator's run-ins with the Bush administration have convinced Keough — a former state senator who was wooed by both camps — that Romney would be a more ideologically reliable Republican standard-bearer. "There are many conservatives who are not enthusiastic about John McCain's candidacy," he said.

Converts such as Keough have pushed Romney to the top ranks of the Republican field. In Romney, who is courting social and religious traditionalists, they see a candidate who can energize Christian conservatives, stay on message and, in Keough's words, "become an acceptable McCain alternative."

His emergence comes after the kind of 2006 that presidential hopefuls dream of. He impressed party leaders with his TV appearances, rolled out a well-received state health plan and saw two key rivals, Sens. Bill Frist of Tennessee and George Allen of Virginia, drop out of the race.

Perhaps more important, Romney avoided the kind of rookie mistakes that can doom a campaign — like the one of his father, former Michigan Gov. George Romney, who self-destructed in 1967 over comments on the Vietnam War.

Now that he has become a contender that political pros see as capable of rubbing sharp elbows with the likes of McCain and former New York Mayor Rudy Giuliani, Romney will undergo much more intense scrutiny than he has received to date.

The searchlight will shine on his record, which has featured policy reversals over the years; his religion — Romney is a Mormon, which troubles some Protestant evangelicals — and his platform, especially his national-security program for the post-Sept. 11 era<.

"A year ago, many people were wondering if he could really get this far," said Julian Zelizer, a politics professor at Boston University. "Once he announces, the cameras will be in front of him and there will be incredible scrutiny," Zelizer said. "We'll see if he survives."

While McCain, 70, has a maverick streak that appeals to independents and the kind of Democrats who used to support President Ronald Reagan, Romney, 59, aims to reconstitute President Bush's coalition. He wants to attract evangelical Christians with his support for a gay-marriage ban, and will try to lure economic conservatives with plans to overhaul health care and the tax system.