The Republican establishment rallied behind Mitt Romney on Wednesday after his Illinois primary win as the party sought to prevent a divisive nominating contest from hurting efforts in the November general election.
Conservative underdog Rick Santorum was unbowed by Romney's widening lead, however, and vowed to take the fight all the way to the Republican convention in August.
With many states awarding delegates proportionately and two other rivals in the race, analysts say it could take Romney until June to win the 1,144 delegates needed to clinch the nomination.
A bloody floor fight -- while unlikely -- is also still possible if Santorum succeeds in eroding the former governor of Massachusetts' air of inevitability.
Former Florida governor Jeb Bush -- the son and brother of two presidents -- threw his considerable weight behind Romney after he swept the Illinois primary with a 12-point lead over Santorum Tuesday night.
"Primary elections have been held in thirty-four states, and now is the time for Republicans to unite behind governor Romney and take our message of fiscal conservatism and job creation to all voters this fall," Bush said in a statement.
"We face huge challenges, and we need a leader who understands the economy, recognizes more government regulation is not the answer, believes in entrepreneurial capitalism and works to ensure that all Americans have the opportunity to succeed."
Senator John McCain, who ran unsuccessfully against Obama in 2008, said that Romney could become the de facto nominee long before he wins the 1,144 delegates needed to clinch the nomination to face President Barack Obama in November.
"I don't know if they will 'drop out' or not," McCain said of Romney's rivals in an interview with AFP.
"But at some point, and I think we may have reached that point in Illinois, the inevitability factor comes into play, so that whether they are still in theoretically or not, that they're no longer hotly contested states."
If enough party elders -- known as super delegates -- pledge their support for Romney he could "very quickly" reach the majority needed to clinch the nomination without having to wait for delegates elected in state primaries, said Steffen Schmidt, a political expert at Iowa State University.
"Then it's over," Schmidt told AFP.
"No more money will flow to (former House speaker Newt) Gingrich who's already dead in the water and Santorum's funding will also freeze."
A protracted battle could also cost Republicans congressional, gubernatorial and state-level seats, especially if Romney isn't able to seal the deal by the time delegate-rich California and four other states hold contests on June 5.
"It may not be possible to win the presidency at that point, but they have to keep the race competitive enough so there isn't a coattail disaster down the ticket," said Larry Sabato, director of the University of Virginia's Center for Politics.
"If you have an incumbent president winning by a wide margin in this polarized era, the odds are he's going to be bringing in a lot of Democrats with him."
Written off in the early days, Santorum, a former Pennsylvania senator, has steadily notched up wins -- 10 out of 33 contests -- largely with the help of evangelicals and the party's most conservative members.
He could regain some momentum with a win in the southern state of Louisiana on Saturday -- where a poll released Tuesday showed Santorum with a 13 point lead over Romney.
Romney's campaign provided Santorum with rich fodder after CNN asked a senior advisor whether the primary had pushed Romney too far to the right for general election voters.
"Well, I think you hit a reset button for the fall campaign. Everything changes," Eric Fehrnstrom said. "It's almost like an Etch A Sketch. You can kind of shake it up and restart all of over again."
Santorum seized on the remark at a campaign stop in Louisiana Wednesday, saying "the reason governor Romney hasn't been able to seal the deal" is because "a lot of voters" know "he will say what he needs to say to win the election."
"We've had enough of politicians like that in this country," Santorum said.
"We need people who whether you like it or not at least you can trust them. At least you know what they're going to do and what they're going to fight for."
Romney's victory in Illinois pushed his delegate count to 560, according to the website Real Clear Politics. Santorum has 246, Gingrich -- who has won two states -- has 141, and libertarian Ron Paul has 66.