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 that Trump would lose in November's election.
Meanwhile retired neurosurgeon Ben Carson, who has performed poorly, has signaled he may drop out of the race.
In the meantime, former nominee Mitt Romney has said he would make a speech later today, in which he is expected to challenge Trump.
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.