Ohio governor John Kasich is leaving the Republican US presidential contest, sources said, giving Donald Trump a clear path to his party's nomination.
Mr Kasich will announce the end of his underdog White House bid on Wednesday, according to three campaign officials.
The decision comes a day after Mr Trump's only other rival, Ted Cruz, dropped out.
With no opponents left in the race, Mr Trump becomes the Republican Party's presumptive presidential nominee to take on the Democratic nominee in November - presumably Hillary Clinton.
Though armed with an extensive resume in politics, the second-term Ohio governor struggled to connect with Republican primary voters in a year dominated by anti-establishment frustration.
Mr Kasich was a more moderate candidate who embraced elements of President Barack Obama's healthcare overhaul and called for an optimistic and proactive Republican agenda.
Even before news of Mr Kasich's decision surfaced, Mr Trump signalled a new phase of his outsider campaign that includes a search for a running mate with experience governing and outreach to one-time competitors in an effort to heal the fractured Republican Party.
"I am confident I can unite much of" the party, Mr Trump said on NBC's Today Show, as several prominent Republicans said they would prefer Democrat Mrs Clinton over the New York billionaire.
In a shot at his critics, Mr Trump added: "Those people can go away and maybe come back in eight years after we served two terms. Honestly, there are some people I really don't want."
His comments on several networks came a few hours after Mr Trump, once dismissed as a fringe contender, became all but certainly the leader of the Republican Party into the autumn campaign against Mr Clinton.
The former secretary of state suffered a defeat in Indiana to her rival, Bernie Sanders, but holds a definitive lead in Democratic delegates who will decide the Democratic nomination.
The Republican competition changed dramatically with Mr Trump's Indiana victory and Mr Cruz's abrupt decision to quit the race. Mr Trump won the Indiana contest with 53.3% of the vote, to Mr Cruz's 36.6% and Mr Kasich's 7.6%, according to unofficial results.
Some Republican leaders remain wary of Mr Trump and have insisted they could never support him, even in a face-off against Mrs Clinton.
"The answer is simple: No," tweeted Nebraska senator Ben Sasse, who has consistently said he could not support Mr Trump.
What is their plan moving forward?
"Prayer," responded Republican strategist Tim Miller, a leader of one of the party's anti-Trump groups. "Donald Trump is just going to have an impossible time bringing together the Republican coalition."
Some conservative leaders were planning a Wednesday meeting to assess the viability of launching a third party candidacy to compete with him in the autumn. Such Republicans worry about Mr Trump's views on immigration and foreign policy, as well as his over-the-top persona.
Hours before clinching victory in Indiana, Mr Trump was floating an unsubstantiated claim that Mr Cruz's father appeared in a 1963 photograph with John F Kennedy's assassin, Lee Harvey Oswald - citing a report first published by the National Enquirer.
Mr Trump defended his reference to the Enquirer article as "not such a bad thing", but the line of attack was the final straw for some Republican critics.
"(T)he GOP is going to nominate for President a guy who reads the National Enquirer and thinks it's on the level," Mark Salter, a top campaign aide to 2008 Republican nominee John McCain, wrote on Twitter. He added Mrs Clinton's slogan: "I'm with her."
On finding a running mate, Mr Trump told MSNBC's Morning Joe that he will "probably go the political route", saying he is inclined to pick someone who can "help me get legislation passed". Mr Trump did not identify any of the names under consideration.
He also said he is hoping to decide within a week how to fund a general election campaign, but said he did not want to accept money from super PACs. He told ABC's Good Morning America that he would begin to accept more political donations.
"I'm really looking at small contributions, not the big ones. I don't want anyone to have big influence over me," he said.
A prominent Cruz donor, Mica Mosbacher, quickly signalled support for Mr Trump and urged others to follow.
"I call on fellow conservatives to unite and support our new nominee Trump," said Ms Mosbacher, widow of a member of George H W Bush's cabinet. "My heart goes out to Cruz, who has a bright future. He did the unselfish thing to drop out."
Vermont senator Bernie Sanders eked out a victory over Mrs Clinton in Indiana, 52.5% to 47.5%. But the outcome will not slow the former secretary of state's march to the Democratic nomination. Heading into Tuesday's voting, Mrs Clinton had 92% of the delegates she needs.
"I know that the Clinton campaign thinks this campaign is over. They're wrong," Mr Sanders said. But Mrs Clinton has already turned her attention to the general election.
She and Mr Trump now plunge into a six-month battle for the presidency, with the future of America's immigration laws, healthcare system and military posture around the world at stake.
While Mrs Clinton heads into the general election with significant advantages with minority voters and women, Democrats have vowed not to underestimate Mr Trump as his Republican rivals did for too long.
Mr Trump later said he would be willing to consider Mr Kasich as a running mate.
He told CNN he would "be interested in vetting John," but added that even as governor, Mr Kasich would be "helpful" with swinging Ohio into the Trump column on Election Day.
Mr Trump added he has a good relationship with Mr Kasich.