WASHINGTON -- Republican leaders searched on Wednesday for a last-chance option that could derail Donald Trump's momentum in the presidential nominating contest that gained strength with his seven Super Tuesday victories.

While a Republican split widened between Trump supporters and the party's more establishment mainstream, Democrats showed increasing cohesion as Hillary Clinton, the former secretary of state, won seven states on the way to regaining her status as the inevitable nominee.

Clinton's dominance with black voters carried her to wins across the South. Still, Vermont Sen. Bernie Sanders picked up wins in his home state and in Minnesota, Oklahoma and Colorado, and he vowed to fight on.

Overshadowed by Trump's wins, Texas Sen. Ted Cruz rolled to a close second in the night's delegate count with a victory in his home state. That showing bolstered his case to be the party's Trump alternative, even as rival Florida Sen. Marco Rubio promised to continue his fight. He hopes to win his home state on March 15.

After a poor showing in Republicans contests so far, retired surgeon Ben Carson effectively pulled out of the race, saying he saw no path forward and announcing he would not be at the next Republican debate on Thursday.

The Republican division represents the party's biggest crisis in years, with the prospect of nominating a presidential candidate it can't control. Some party leaders are considering the once unthinkable option of aligning behind the conservative firebrand Cruz, whom many dislike. Others are talking of a contested convention, where none of the candidates has won sufficient delegates in primaries and caucuses to assure nomination.

Some influential power brokers even raise the option of forming a new party.

Republican leaders also fear that a Trump nomination could damage party incumbents in the Senate who could be voted out of office in swing states known to support either major party, costing Republicans their majority in Congress.

An Associated Press delegate count indicates Trump will have to do better in future contests to claim the nomination before the party's July convention. So far, he has won 46 per cent of the delegates awarded, and he would have to increase that to 52 per cent in the remaining primaries.

The next round of voting in a busy March comes Saturday, with Louisiana's primary, Republican caucuses in Kentucky and Maine, a Democratic caucus in Nebraska and caucuses for both parties in Kansas.

Trump won handily in Super Tuesday primary and caucus states as politically opposite as Massachusetts and Alabama, a sign of his broad, outsider appeal.

Along with Texas, Cruz took neighbouring Oklahoma as well as Alaska. Rubio won only liberal Minnesota.

Despite Trump's victories, many Republican leaders remained deeply skeptical he could beat Clinton in a head-to-head matchup in November.

"Ted Cruz is not my favourite by any means," South Carolina Sen. Lindsey Graham told CBS. "But we may be in a position where rallying around Ted Cruz is the only way to stop Donald Trump, and I'm not so sure that would work."

The comments came as the .NeverTrump hashtag spread across Twitter.

Mitt Romney, the Republican nominee four years ago, announced plans to speak on the "state of the 2016 presidential race" Thursday in Utah. The former Massachusetts governor has moved aggressively to take on Trump in recent days, saying the billionaire's unreleased tax returns might contain "bombshells." However, he was not expected to endorse a candidate or announce a late entry into the race himself.

In his victory speech, Trump sent a clear message to the Republican establishment, warning House Speaker Paul Ryan, who had declared earlier Tuesday that "this party does not prey on people's prejudices," that if the two don't get along, Ryan is "going to have to pay a big price."

Trump won at least 234 delegates on Tuesday, and Cruz won at least 209. Rubio was a distant third with at least 90. There were 595 Republican delegates at stake in 11 states. There were still 40 delegates left to be allocated. Securing the nomination requires 1,237 delegates.

Overall, Trump leads with 316 delegates and Cruz has 226. Rubio has 106 delegates, Ohio Gov. John Kasich has 25 and Carson has eight.

The math was also tough for Sanders. Clinton was assured of winning at least 457 of the 865 delegates at stake Tuesday. Sanders gains at least 286. When including party leaders, Clinton has at least 1,005 delegates and Sanders has at least 373. It takes 2,383 delegates to win the nomination.