WASHINGTON — From his outspoken defense of President Bush on interrogating terrorism suspects to an appeal to Christian conservatives here Friday, Gov. Mitt Romney is increasingly trying to position himself as the leading conservative alternative to Sen. John S. McCain, R-Ariz., in the 2008 presidential race.

Romney, elected just four years ago as a moderate Republican, is seizing on the opening created by an unexpectedly tough re-election campaign this year for Sen. George F. Allen of Virginia, the prospective presidential candidate who had been widely seen as the standard-bearer for the GOP's conservative wing.

As Allen has been forced to put his 2008 hopes on hold, analysts and observers say, Romney has rushed to fill the vacuum by appealing to the right on homeland security, immigration and social issues, particularly in South Carolina, a crucial early primary state home to many conservative voters.

"He's maximized his window of opportunity very well in South Carolina," said Scott Malyerck, executive director of the state Republican Party. "He's hitting those conservative buttons that folks are concerned about."

On his blog this week, conservative writer Andrew Sullivan called Romney the "theo-cons' favorite nominee" who has "clearly positioned himself as McCain's opponent on the issue" of interrogation methods.

"I presume he thinks it will win him votes among Christian, Republican base voters," Sullivan wrote, citing two often interconnected constituencies in the Republican Party.

Though McCain is still considered the front-runner for the Republican nomination, his independent streak on issues such as immigration and tighter campaign finance rules has long alienated some conservatives. With the 2008 jockeying intensifying, Romney is trying to exploit that rift by articulating positions that fall to the right of McCain's and by courting religious conservatives.

When McCain and two other Republican senators broke with Bush last week in insisting on greater rights for terrorism suspects in American custody, Romney firmly backed the president, labeling the move by McCain and the other maverick senators "a big mistake." Romney told The New York Times he was "foursquare behind the president" and went out of his way to cite several other areas, including immigration policy, where he held more conservative views than McCain.

Romney's criticism drew a curt response from McCain, but it has won plaudits elsewhere.

"Romney gets it, and McCain hasn't got a clue," reads a blog entry on gopbloggers.org.

Republicans in South Carolina, Malyerck said, appreciate Romney's support for the president, despite the fact that one of those bucking Bush was Lindsey Graham, the state's senior senator.