(Reuters) - Retired neurosurgeon Ben Carson signalled on Wednesday he was quitting the Republican presidential race, leaving three candidates facing Donald Trump as the party establishment struggled to find a way to halt the outspoken businessman.
Carson, a conservative who briefly led opinion polls among Republicans earlier in the campaign, said he did not "see a political path forward" after performing poorly in this week's Super Tuesday nominating contests. He said he would not participate in a Republican debate on Thursday.
Despite a push by some mainstream Republicans to try to block Trump from winning the nomination, the billionaire industrialist Koch brothers will not deploy their $400 million political arsenal to attack him in the presidential primary campaign.
As a conservative black Republican, Carson, 64, stood out in the mostly white Republican Party, but his campaign foundered amid staff infighting and questions about Carson’s familiarity with foreign policy.
His departure is unlikely to have a major impact on the fight among Republicans to become the party's candidate in the Nov. 8 election to succeed Democratic President Barack Obama.
Reuters/Ipsos polling last month showed Carson supporters would mostly likely be split if he dropped out between Trump and U.S. Senators Marco Rubio of Florida and Ted Cruz of Texas.
Trump consolidated his lead in the Republican race with a string of victories on Tuesday that moved him closer to becoming the nominee. The 69-year-old New York real estate tycoon won seven states from Massachusetts to the conservative Deep South.
His victories compounded the problem for a party whose leaders are critical of many of Trump's positions and values and sceptical he can defeat the likely Democratic nominee, former Secretary of State Hillary Clinton.
Trump has showered insults on rivals and is facing strong party disapproval over his ideas to build a wall between the United States and Mexico, deport 11 million illegal immigrants and temporarily bar Muslims from entering the country.
The 2012 Republican nominee, former Massachusetts Governor Mitt Romney, plans to deliver a rebuke of Trump on Thursday in a speech in Utah, sources familiar with his thinking said. It will be a high-profile display of establishment Republican unease.
Romney's speech comes on the same day that Trump and his remaining rivals, Rubio, Cruz and Ohio Governor John Kasich share a debate stage in Detroit, hosted by Fox News.
The Koch brothers, the most powerful conservative mega donors in the United States, "have no plans to get involved" in the Republican primary process, James Davis, spokesman for Freedom Partners, the brothers’ political umbrella group, told Reuters.
Donors and media reports have speculated that the brothers would launch a "Trump Intervention," a strategy that would involve deploying the Kochs’ vast political network to target Trump in hopes of removing him from the race.
Trump has reached out to House of Representatives Speaker Paul Ryan, the most senior Republican in Congress. The Trump campaign contacted Ryan's office late on Monday, a day before the speaker publicly admonished Trump over his failure to repudiate the backing of his candidacy by a white supremacist group. Trump responded with a warning to Ryan.
Among Trump's rivals, Cruz, 45, won three states on Tuesday, bolstering the conservative senator's argument that he has the best chance of stopping former reality TV star Trump.
The Republican establishment's favoured 2016 candidate, Rubio, only won one Super Tuesday state, taking Minnesota.
Anti-Trump Republicans have yet to coalesce around a single strategy to halt him, but the conservative group Club for Growth claimed credit for slowing Trump in some primary states by running attack ads. It said it would air a new advertisement in Florida as part of a $1.5 million ad buy.
Some party donors - including hedge-fund manager Paul Singer and Meg Whitman, the Hewlett-Packard Enterprise chief executive - organised a phone call on Tuesday to get funding for an anti-Trump effort, The New York Times reported.
But one of Trump's former rivals in the 2016 race, Mike Huckabee, admonished Republicans for not respecting the will of the voters.
"The establishment Republicans are all bed-wetting over this and they don’t seem to understand that we have an election," the former Arkansas governor said on Fox News. "Let’s remember that we have an election process, not a selection process."
Trump responded to the furore against him, saying in a tweet on Wednesday: "The special interests and people who control our politicians (puppets) are spending $25 million on misleading and fraudulent T.V. ads on me."
Trump later unveiled proposals for reforming U.S. healthcare that included repealing Obamacare, allowing prescription drugs to be imported, and turning the Medicaid program for the poor into block grants to states.
Democrats pounced on the Republican campaign infighting.
Senate Minority Leader Harry Reid called Trump a "monster" the Republicans spawned with their years of rancorous opposition to all major Obama administration initiatives.
"Republicans created him by spending seven years appealing to some of the darkest forces in America," Reid said on the Senate floor.
In the Democratic race, Clinton, 68, took big steps on Tuesday toward securing her party's nomination, the 2016 campaign's biggest day of state-by-state nominating contests.
Clinton's rival, U.S Senator Bernie Sanders, 74, won his home state of Vermont along with Colorado, Minnesota and Oklahoma.
Reporting by John Whitesides in Washington and Steve Holland in Detroit; Additional reporting by Ginger Gibson, Richard Cowan, Doina Chiacu, Megan Cassella and Eric Walsh in Washington and Michelle Conlin in New York; Writing by Doina Chiacu and Alistair Bell; Editing by Frances Kerry and Peter Cooney