A plan to block Republican presidential candidate Donald Trump struggled to gain traction on Friday as rival candidates rejected it, while Democrats revelled in the chaos they hoped would boost their chances of keeping the White House.

The country's top elected Republican, House of Representatives Speaker Paul Ryan, said he was not interested in an effort to draft him into the White House race.

And U.S. Senator Ted Cruz, a conservative presidential hopeful, ruled out a deal to pick a compromise Republican candidate at the party's July convention, which senior party figures see as their best chance to stop the unpredictable billionaire.

"The D.C. power brokers will drop someone in who is exactly to the liking of the establishment. If that will happen we will have a manifold revolt in this country," Cruz said at the Conservative Political Action Conference outside Washington.

"You want to beat Donald Trump, you beat Donald Trump with the voters," he said.

Party leaders worry Trump would not be able to beat Democratic front-runner Hillary Clinton in the election, but time is running out after he won most of the states that voted in this week's Super Tuesday.

Senior Republicans also fear Trump's plans to build a wall on the U.S.-Mexican border and ban Muslims from entering the United States will turn off voters in November and upset U.S. allies.

Others note his past support for liberal policies and question whether he has any agenda other than advancing himself.

"I don't think he actually carries the conservative mantle. He's a little too crass for me," said Michele Minter, a San Diego executive assistant who was attending CPAC.

Trump, a former reality TV star, often plays by his own set of rules. He cancelled plans to speak at CPAC, normally an essential stop for ambitious Republicans, and will instead attend a rally in Kansas.

The real estate magnate, who is drawing support from many blue-collar Republicans concerned about illegal immigration and stagnant wages, has won most Republican nominating contests and leads in many polls for the primary contests still to come.

"I'm not a normal Republican," he said to huge cheers at a rally in Warren, Michigan.

Retired neurosurgeon Ben Carson, the only candidate to ever challenge Trump's months-long lead in opinion polls, officially ended his White House bid.

"There are a lot of people who love me, they just won't vote for me," Carson said in a speech at CPAC, held in National Harbor, Maryland.

RYAN 'NOT INTERESTED'

A new group called the Committee to Draft Speaker Ryan filed papers with the Federal Election Commission on Thursday, seeking to raise money to push Ryan as a Republican alternative.

Ryan, a budget wonk who was the Republican vice presidential candidate in 2012, is seen by many in the party as a unifier after he took the speaker's job last year to unite establishment Republican lawmakers and conservative upstarts in the House.

"He is flattered, but not interested," Ryan spokeswoman AshLee Strong said in an email on Friday.

As Trump cements his front-runner status, senior party figures hope to deny him enough delegates to clinch the nomination, which would give them the chance to choose a compromise candidate at their convention in Cleveland.

The last time that happened at a Republican convention was in 1948 when Thomas Dewey was nominated.

Republican National Committee Chairman Reince Priebus said there was an 85 to 90 percent chance that the party will not face that scenario this year.

Mitt Romney and John McCain, the party's last two presidential nominees, called on Republicans to halt Trump's rise by backing whichever candidate was strongest in their state, a form of tactical voting.

Few elected officials are rallying behind the "Dump Trump" banner. The party's 31 state governors, for example, are not lining up behind an alternative. Only five have endorsed Florida Senator Marco Rubio and one has backed Cruz, in a sharp contrast to previous years when governors overwhelmingly endorsed the party's eventual nominee.

Rubio has only one state so far and is gearing up for what could be a make-or-break contest in his home state on March 15. Cruz said Saturday he planned to open 10 campaign offices there, in what could be an effort to force the rival senator out of the race.

Trump is expected to extend his lead on Saturday, when a total of 155 delegates are at stake in Kansas, Louisiana, Maine and Kentucky.

Democrats were happy to let Republicans fight amongst themselves. "We can sit back and let them light their own dumpster fire and wait until they're finished," said Eddie Vale, spokesman for American Bridge, a Clinton-allied group which collects negative research on Republican candidates.

"They're giving us so much great video footage that we could run ads between now and November of nothing but Republicans attacking Trump," Vale told Reuters.

Nationally, Trump has the support of 41 percent of Republican voters, compared to 19 percent who back Cruz and 16 percent who back Rubio, according to Reuters/Ipsos polling data.