By Stephen Collinson

(CNN) -- Donald Trump is the presumptive Republican presidential nominee following a decisive victory in the Indiana primary and the decision by Ted Cruz to drop out of the race.

Though Trump has not formally secured the 1,237 delegates he needs to win the nomination -- and likely won't until June -- there is no serious opposition left to block his path.

"It is a beautiful thing to watch, and a beautiful thing to behold," Trump said during a victory speech. "We are going to make America great again."

Cruz tried everything to pull off a last-ditch win in Indiana, including the unusual move of selecting Carly Fiorina as his running mate even though he wasn't the nominee. He also forged a pact with John Kasich that would allow him to focus on Indiana while the Ohio governor would devote his time to later states.

But none of the moves worked.

"We left it all on the field in Indiana. We gave it everything we've got but the voters chose another path," Cruz said. "So with a heavy heart but with boundless optimism for the long-term future of our nation, we are suspending our campaign."

Following Cruz's speech, Republican National Committee Chairman Reince Priebus tweeted Trump is now the presumptive nominee and encouraged the party to "unite and focus on defeating" Hillary Clinton.

Trump's candidacy has galvanized the GOP, bringing in voters -- especially in regions like the Rust Belt -- that might not otherwise be attracted to the party's message. In the process, he's toppled a GOP field that, at the start, included many well-respected governors and senators.

GOP elites now face the long-feared reality of Trump as an outsider nominee who will lead them into the fall campaign after splitting the party, overturning establishment and conservative power bases and alienating key general election voters with incendiary rhetoric.

The anti-Trump movement said it would fight on as Trump was still short of the delegates needed to secure the nomination. Katie Packer, the chair of Our Principles PAC, said there is still time for Trump to "continue to disqualify himself in the eyes of voters."

"We continue to give voice to the belief of so many Republicans that Trump is not a conservative, does not represent the values of the Republican Party, cannot beat Hillary Clinton, and is simply unfit to be President of the United States," she said in a statement.

For his part, Kasich insisted he would remain in the race.

"Tonight's results are not going to alter Gov. Kasich's campaign plans," said John Weaver, Kasich's chief strategist. "Our strategy has been and continues to be one that involves winning the nomination at an open convention."

Sanders wins Indiana

Meanwhile, Bernie Sanders will win the Indiana Democratic primary, according to a CNN projection. That victory could help boost his campaign's morale but is unlikely to cut deeply into Clinton's large delegate lead that has her on track for the nomination.

Trump's dramatic victory caps a day of extraordinary developments in the Republican race.

Cruz, facing the prospect of an Indiana defeat, snapped after weeks of personal attacks from Trump that included fresh insinuations that his father was associated with Lee Harvey Oswald.

"I'm going to tell you what I really think of Donald Trump," he told reporters at a morning news conference.

Cruz blasted Trump as a "pathological liar," "utterly amoral," "a narcissist at a level I don't think this country's ever seen" and "a serial philanderer."

He unleashed his full arsenal of insults in his attack on Trump.

"He is proud of being a serial philanderer ... he describes his own battles with venereal diseases as his own personal Vietnam," Cruz said, citing a decades-old Trump appearance on "The Howard Stern Show."

"This man is a pathological liar, he doesn't know the difference between truth and lies ... in a pattern that is straight out of a psychology textbook, he accuses everyone of lying," Cruz said as Indiana voters headed to cast their ballots. "Whatever lie he's telling, at that minute he believes it ... the man is utterly immoral," Cruz said.

Trump hit back in a statement blasting Cruz as a "desperate candidate trying to save his failing campaign."

"It is no surprise he has resorted to his usual tactics of over-the-top rhetoric that nobody believes," Trump said. "Over the last week, I have watched Lyin' Ted become more and more unhinged as he is unable to react under the pressure and stress of losing, in all cases by landslides, the last six primary elections --- in fact, coming in last place in all but one of them.

Trump added: "Today's ridiculous outburst only proves what I have been saying for a long time, that Ted Cruz does not have the temperament to be President of the United States."

Increasingly personal battle

The volley reflected the increasingly personal battle between Cruz and Trump in the final days of the Indiana contest. Earlier Tuesday, Trump had criticized Rafael Cruz, the senator's father, calling him "disgraceful" after he urged evangelical voters in Indiana to reject his son's rival.

Trump also referenced a report from the tabloid National Enquirer -- without naming the publication -- which alleged that it had identified Rafael Cruz in a photo with Lee Harvey Oswald months prior to the JFK assassination. CNN has not independently confirmed that report -- and there is no evidence that it is true.

"And (Ted Cruz's) father, you know, was with Lee Harvey Oswald prior to Oswald's, you know, being shot. I mean the whole thing is ridiculous," Trump said in an interview on "Fox and Friends." "I mean what was he doing with Lee Harvey Oswald, shortly before the death? Before the shooting? It's horrible."

Trump went into Tuesday leading the GOP field with 1,002 delegates and needing to win just 47% of the 502 remaining delegates on offer. Cruz has 572 delegates and Kasich has 156. Neither man can now reach 1,237 delegates.

Indiana is not a classic winner-take-all contest on the Republican side. To win all 57 Republican delegates, a candidate must win the statewide popular vote and the popular vote in all nine of the state's congressional districts.

Democratic race

Going into the Indiana primary, Clinton has 2,179 of the 2,383 delegates needed to win the nomination, including superdelegates -- senior party officials and lawmakers who can vote however they choose at the convention. She only needs to win 20% of the remaining delegates at stake to formally capture the nomination.

Sanders has 1,400 delegates so far, including superdelegates, and he needs to capture 97% of the remaining delegates to overhaul Clinton. There are 1,016 delegates remaining to be contested in the campaign. Clinton leads Sanders in the pledged delegate count by 1,666 to 1,359 and by 513 superdelegates to Sanders' 41.

The painful state of the race for Sanders means that his only hope of winning the nomination would be to persuade Democratic superdelegates at the convention to abandon the former secretary of state -- an unlikely scenario considering that group is largely made up of party insiders long aligned with Clinton.

That is one reason why the Clinton campaign has begun stressing that it is time to unite the party and concentrate on the race against Republicans in the fall.

But the Vermont senator used his public performances in Indiana to highlight what he says are stark differences on trade. He also rebuked her over her vote in 2002 to authorize the Iraq war, and demanded a $15 minimum wage.

"Let Indiana be the 18th state to join the political revolution!" Sanders said at a rally in Fort Wayne, Indiana, on Monday.

CNN's Chris Frates and Eric Bradner contributed to this story
   
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