Trump to GOP: I'm your nominee now

"We're going after Hillary Clinton," said Trump, who is now the likely GOP nominee. "She will not be a great president. She will not be a good president. She will be a poor president."

Trump rolled over Ted Cruz in Indiana, extinguishing Cruz's last hope to emerge as the only viable alternative to Trump as the race heads toward the GOP convention in Cleveland.

Previewing attacks to come, Trump attacked Clinton on coal and trade, and went after her husband's implementing of the NAFTA in the 1990s.

"Her husband singed perhaps in the history of the world, the single worst trade deal ever done," he added. "It's called NAFTA, and I was witness to the carnage, over the last six weeks especially."

Trump took the stage shortly after Cruz conceded from the race, congratulating the Texas senator on being a fierce competitor with a bright future.

"You know we started off with that 17 number, and just so you understand, Ted Cruz, I don't know if he likes me or doesn't like me, but he is one hell of a competitor," Trump said.

Tuesday night's conciliatory tone was a departure from Trump's usual mockery.

Earlier in the day, Trump peddled a spurious story from the National Enquirer claiming that Rafael Cruz, Cruz's father, was an associate of Kennedy assassin Lee Harvey Oswald.

Trump also has infamously attack Cruz's wife on the basis of her looks and propagated the unflattering nickname of "Lyin' Ted" to describe his Republican opponent -- a epithet he was using on Twitter even after the polls had closed in Indiana.

"He's got an amazing future so I want to congratulate Ted, and I know how tough it is," Trump said in his speech. "It's tough. I've had some moments where it was not looking so good, and it's not a great feeling, and so I understand how Ted feels and Heidi and their whole beautiful family. And I want to just say though -- one tough competitor and I can say that for the others."

Moments before Trump took the stage, Reince Priebus, the chairman of the Republican National Committee, tweeted that the party should rally around Trump as their standard bearer and "presumptive nominee."

Trump, who had already declared himself the presumptive nominee last week after victories in Northeast and Mid-Atlantic states, told the press gathered in the lobby of Trump tower that he had spoken to Priebus over the phone prior to taking the stage to thank and congratulate him.

"He's doing a tremendous job," Trump said. "It's not an easy job when he had 17 egos. Now I guess he's down to one. I don't know is there a second? Is there a second? I don't know. I'm going to have to ask you folks to explain the status of that."

There was no mention of Gov. John Kasich's name during Trump's 20 minutes speech. Kasich, the last remaining Republican presidential contender other than Trump, says he is planning to stay in the race.