By Ginger Gibson
MONROE, N.C. (Reuters) - Republican leaders opposed to Donald Trump becoming the party's U.S. presidential candidate warned on Wednesday that the billionaire businessman could become unstoppable if he wins two crucial nominating contests next week.
One of Trump's fiercest critics in the Republican Party, U.S. Senator Lindsey Graham, said that if the billionaire businessman extends his winning streak into crucial contests next week, the New Yorker's path to the nomination was all but inevitable.
"I'm worried that if we lose Florida and Ohio he may be unstoppable," said Graham, a former Republican candidate who was among the party's large field of early White House hopefuls.
With victories in nominating contests on Tuesday in three out of four states, Trump maintained his lead over the three remaining challengers in the race to be the Republican candidate in the Nov. 8 presidential election.
On the Democratic side, Hillary Clinton's surprise loss to Bernie Sanders in the important Michigan primary laid bare growing voter anger over trade, raising warning flags for her ahead of a possible election showdown against Trump.
On Wednesday, the often-combative Trump struck a more conciliatory tone toward the Republican establishment that has fiercely resisted his advance - first by backing favorite candidates who failed to win votes, then by pouring money into campaigns against him.
"If we embrace what is happening and everyone came together, instead of spending all this money on these ads ...," Trump told Fox News. "If everyone came together, no one could beat the Republican Party. We would walk into Washington."
He also praised U.S. House of Representatives Speaker Paul Ryan after speaking with him by phone this week, calling the Republican leader a "good man."
Trump, 69, fended off a week of attacks from the party's establishment and defied predictions his campaign might be losing steam with his primary wins on Tuesday in Michigan, the biggest prize of the night, as well as Mississippi and Hawaii. His closest challenger, Senator Ted Cruz of Texas, won Idaho.
Trump's convincing win in Michigan narrowed prospects for the party's anti-Trump forces to stop him before next Tuesday's contests, which also include Illinois, North Carolina and Missouri.
Trump now has 458 of the 1,237 delegates needed to win the nomination, with Cruz second with 359 delegates, according to the Associated Press. Rival Marco Rubio trails with 151 delegates, while John Kasich is far behind with 54.
ENDORSEMENTS: ANY ONE BUT TRUMP
Cruz, with enough primary victories to present himself as a viable Trump alternative, won the endorsement of former Republican rival Carly Fiorina on Wednesday and appealed to anti-Trump Republicans to back him.
"If you don't want to see Donald Trump as the nominee, if you don't want to hand the election on a silver platter to Hillary Clinton and the Democrats, then I ask you to join us," Cruz said.
Trump, a former reality TV show host, has peppered his campaign with put-downs of rivals and critics. Many mainstream Republicans have been offended by his statements on Muslims, immigrants and women and alarmed by his threats to international trade deals.
But Cruz, 45, is a hard alternative for some of the mainstream Republican leadership to get behind. A fiscal and social conservative, he is unpopular with fellow senators who were targets of his personal attacks and who blame him for orchestrating a government shutdown in 2013 in a failed attempt to derail President Barack Obama's healthcare reforms.
In January, Graham called a choice between Trump and Cruz like "being shot or poisoned." But the senator softened his tone on Wednesday, saying it had become increasingly clear that "Ted Cruz is the most viable alternative to Donald Trump."
An aide to former Florida Governor Jeb Bush, who dropped out of the race last month, said Bush would meet with Cruz, fellow Floridian Rubio, and Kasich, governor of Ohio, in Miami before the March 15 contests. It was not clear if he planned to make an endorsement.
Clinton, 68, won in Mississippi, but the victory in Michigan by her opponent from Vermont was expected to prolong the fight to pick a candidate.
Clinton holds a sizable delegate lead, with 1,221 compared with Sanders' 571. She has been helped by winning Southern states by big margins and keeping most of her losses narrow.
Clinton's campaign has been dogged by questions over her use of a private email server while she was secretary of state and - in a sign the issue will not go away - the Republican National Committee filed a lawsuit on Wednesday seeking emails between Clinton and her aides.
Trump could open a sizable delegate lead if he is able to win next Tuesday in Florida, Ohio, or Illinois, states that allot all their delegates to the winner.
Kasich, 63, of Ohio, and Rubio, 44, of Florida, could throw a wrench into Trump's delegate stash if they win in their home states next week.
(Additional reporting by Steve Holland, Susan Heavey, Doina Chiacu, Amanda Becker in Washington; Writing by Doina Chiacu and Jonathan Oatis; Editing by Frances Kerry and Howard Goller)