Senior Republicans are speaking out against front-runner Donald Trump after his seven-state victory in the race for the party's presidential pick.
Trump has declared himself a "unifier", but he is strongly opposed by veteran Republican politicians.
Senator Lindsey Graham warned on Wednesday that Trump would lose in November's election.
Meanwhile, retired neurosurgeon Ben Carson, who has performed poorly, has signalled he may drop out of the race.
In a statement on Wednesday, he said he saw "no path forward" for his presidential campaign and will not attend Thursday's TV debate.
But he stopped short of ending his race completely.
Meanwhile, former nominee Mitt Romney has said he would make a speech on Thursday in which he is expected to challenge Trump.
He has been fiercely critical of the businessman, whose hardline stance on issues like immigration put him at odds with Republican orthodoxy.
His victories on so-called Super Tuesday consolidated his position as the most likely Republican candidate to vie for the White House against the Democratic nomination.
Texas Senator Ted Cruz walked away with three states, bringing his total to four, while Florida Senator Marco Rubio won his first state (Minnesota) in the primary race to date.
Several party leaders, including House Speaker Paul Ryan and South Carolina Senator Lindsey Graham, have spoken out against Donald Trump's controversial policies and positions in recent days.
His latest controversy centres on his failure to disavow David Duke, a leader of the white supremacist Ku Klux Klan, who endorsed him. He later said he had on several occasions in the past disavowed Duke.
Ryan forcefully denounced the real estate mogul on Tuesday, saying: "They must reject any group or cause that is built on bigotry. This party does not prey on people's prejudices."
"Let me make it perfectly clear, Senate Republicans condemn David Duke and the KKK, and his racism," Senator and majority leader Mitch McConnell commented.
Influential congressman Peter King joked he would leave politics if Trump became the nominee.
And Marco Rubio indicated in his speech on Tuesday that the Republican establishment was unlikely to back the former reality TV star.
"If this was anybody else as a front-runner, there'd be people right now saying 'Let's all rally around the front-runner,'" he said, adding, "that will never happen with Donald Trump".
According to the New York Times, some party donors are already trying to raise funds for an anti-Trump effort.
Meanwhile, The Center for Public Integrity reports that Republican super PACs ran as many as 8,500 adverts in the run up to Super Tuesday to try to discredit Donald Trump's election campaign.
Graham warned that Republicans would "lose to Hillary Clinton" with Trump as their party nomination.
"We may be in a position where we have to rally around Ted Cruz as the only way to stop Trump," he said. "And I'm not so sure that would work."
Clinton also won seven states on Tuesday, consolidating her lead in the Democratic race over rival Bernie Sanders.