Mitt Romney has secured a key endorsement for his presidential bid from former Florida Governor Jeb Bush.
Mr Bush, whose support comes the day after Mr Romney secured a clear victory in Illinois, called on Republicans to "unite" behind the Romney campaign.
Correspondents say the backing of the respected Republican leader suggests the party establishment could be coalescing around Mr Romney.
The eventual nominee will challenge Barack Obama in November's election.
"Primary elections have been held in 34 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," Mr Bush said in a statement.
"I am endorsing Mitt Romney for our party's nomination.
"We face huge challenges, and we need a leader who understands the economy, recognises more government regulation is not the answer, believes in entrepreneurial capitalism and works to ensure that all Americans have the opportunity to succeed."
Mr Bush had been courted as a possible presidential candidate himself, and his father, former President George H W Bush, has also effectively endorsed Mr Romney's candidacy.
On Tuesday, FreedomWorks, an organisation that supports the Tea Party movement, dropped its opposition to Mr Romney's candidacy.
"It is a statistical fact that the numbers favour Mitt Romney," Russ Walker, vice-president of FreedomWorks told the Washington Times.
"We are dedicated to defeating Obama and electing a conservative Senate that will help Romney repeal Obamacare [healthcare reform] and address the nation's economic and spending challenges."
Mr Romney has consistently had difficulty winning over the Republican base, who have questioned his conservative credentials.
'Competing every state'
On Tuesday, Mr Romney secured 47% of the Illinois vote, with a comfortable lead of 12 points over Rick Santorum, his closest rival.
Ron Paul polled 9% in Illinois and Newt Gingrich was on 8%; neither candidate campaigned extensively in the state.
"I'm running for president because I have the experience and the vision to get us out of this mess," said Mr Romney, as his victory became evident.
However, addressing supporters on Tuesday evening, Mr Santorum said he had polled well in Illinois in areas "that conservatives and Republicans populate".
"We're very happy about that and we're happy about the delegates we're going to get, too," he said.
A candidate needs to accumulate 1,144 delegates to the August convention in order to secure the nomination.
Analysts say the current figures make that an almost impossible task for Mr Santorum, who has spoken openly in recent weeks about winning enough delegates to stop Mr Romney taking the crown.
Such an outcome would lead to a competitive vote at the Tampa convention, in which Mr Santorum feels he could overcome Mr Romney.
Mr Santorum has vowed to continue, "competing in every state", citing tepid support for Mr Romney even in states the former Massachusetts governor has won.