"Giuliani's endorsement of McCain only reinforces what voters were likely to do on their own anyway," Scala said.

The two were competing for support of the same Republican moderates, according to Charles Bierbauer, dean of the college of journalism at the University of South Carolina.

Former Massachusetts Gov. Mitt Romney still must contend with former Arkansas Gov. Mike Huckabee for support among the GOP's conservative base.

The immediate effect of the smaller field of presidential hopefuls means fewer participants in televised debates.

That's a benefit to Obama, who has become the sole alternative to Clinton, whom some voters see as a symbol of the Washington political establishment by virtue of the two terms her husband, Bill, served as president.

"It does make it a one-on-one contest, which I think is advantageous to the challenger because the anti-Clinton vote won't be divided anymore," Scala said. "Obama has a clear shot now. He doesn't have to worry about Edwards dogging him."

For Clinton, the plus is that Edwards drew much of his support from among blue-collar white voters in South Carolina. That same group might vote for her in upcoming Southern primary states such as Alabama, Arkansas, Georgia, Kansas, Missouri, Oklahoma and Tennessee.

"Is that something that accrues to Clinton?" asked Bierbauer. "Conventional wisdom would suggest that."

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Contributing: Susan Page, USA Today

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E-mail: btumulty@gns.gannett.com