Will it be Texas Gov. Rick Perry, who's backsliding in recent polls.
Or will front-runner former Massachusetts Gov. Mitt Romney hang on to with the GOP nomination?
Romney was looking to pick up support from Republican activists who had been poised to back Christie. A handful of major Republican donors jumped into Romney's camp after Christie on Tuesday ruled out a candidacy.
But many party activists still appear restless, casting about for a conservative alternative and wondering if Perry can fill the role despite his shaky debate performances.
Polls show that Romney attracts about one-fourth of prospective Republican voters, with the rest looking to Perry, businessman Herman Cain and others.
Few campaign veterans think Cain, who has never held elected office, can win the nomination. But his rise, similar to the one once made by Rep. Michele Bachmann, signals that many Republican activists still prefer someone more aggressively conservative than the measured Romney, who ran in 2008 and has long been seen as the party's establishment candidate.
Cain "is starting to pick up steam, more so than I'm hearing for Perry," said Glenn McCall, Republican chairman in York County, South Carolina.
Brendan Steinhauser of FreedomWorks, a group linked to the tea party movement, said Cain's rise "shows the opening for a conservative is still there." Either Cain or Perry "will likely emerge as the conservative, anti-establishment alternative to Mitt Romney," he said. "The final goal is beating Barack Obama with the most conservative candidate that can win."
Romney's supporters say he has the best chance of ousting Obama. They point to Perry's debate problems, and to questions about the racist name of a Texas hunting camp Perry has used, as typical of the surprises that bring fast-soaring contenders back to earth.
Yet Romney has long struggled to win Republican voters' enthusiasm.
"Nobody wants to put a candidate forward just because they happen to be the most electable," said veteran campaign consultant Terry Nelson. Voters want someone "who has the kind of vision and solutions they think might work," he said, and Romney's team is "trying to put forward that vision."
Perry showed his impressive fundraising Wednesday, when his campaign reported raising more than $17 million in his first seven weeks as a candidate. For the quarter that just ended, Romney was expected to raise less than the $18 million he brought in during his first three-month fundraising period. Paul, the libertarian-leaning Texas congressman, said he raised about $8 million.
Obama, meanwhile, has stepped up his fundraising as well as his jabs at his Republican opponents. He recently described the Republican contenders as "a stage full of political leaders" who failed to admonish a Republican debate audience that booed a gay soldier stationed in Iraq.
Some had expected Palin to come in and draw heavy conservative support.
Sen. John McCain plucked Palin from relative obscurity in 2008 by naming her as his running mate. She electrified Republican activists for a while, delivering a well-received speech at the Republican national convention.
But Palin later seemed overwhelmed by the national spotlight, faltering at times in televised interviews even when asked straightforward questions.
Palin on Wednesday told conservative radio host Mark Levin that she would not consider a third party candidacy because it would assure President Barack Obama's re-election.
In a video posted on YouTube, Palin said, "you don't need an office or a title to make a difference."