Senior Republicans are speaking out against front-runner Donald Trump after his seven-state victory in the race for the party's presidential pick.
Mr Trump has declared himself a "unifier", but he is strongly opposed by veteran Republican politicians.
Senator Lindsey Graham warned on Wednesday that Mr 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 Mr 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.