The chairman of the Republican National Committee has declared Donald Trump as the party's "presumptive" presidential nominee after the real estate tycoon swept the Indiana primary, prompting his closest rival Sen. Ted Cruz to drop out of the race.

In making the declaration on a Twitter post on Tuesday night moments after Trump secured his victory in Indiana, Chairman Reince Priebus also called on the party to unite and focus on defeating likely Democratic presidential nominee Hillary Clinton in November, Newsmax reports.

In his victory speech, Trump praised Cruz as a "tough, smart guy" right after the Texas senator announced that he's suspending his campaign.

"I have met some of the most incredible competitors that I have ever competed against right here in the Republican Party," Trump said to cheering supporters at Trump Tower in New York City.

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

"He is a tough, smart guy," Trump added. "And he has got an amazing future.

"So, I want to congratulate Ted. I know how tough it is. It's tough. It's tough. I've had some moments where it was not looking so good — and it's not a great feeling.

"I understand how Ted feels and Heidi and their whole beautiful family," he added. "I want to just say, though, that's one tough competitor."

Trump's conciliatory tone, however, did not match that of Cruz whose concession speech did not give any inkling that he would eventually endorse Trump for the sake of party unity.

The division came into sharp focus even before the Indiana votes were counted on Tuesday night, WND reports.

Commentators said the differences between Trump and Cruz may now be irreconcilable following the latest firestorm triggered by a National Enquirer article that contained a photo allegedly showing a young Rafael Cruz, Ted's father, handing out pro-Fidel Castro pamphlets in New Orleans in 1963 alongside Lee Harvey Oswald, the man believed to have killed John F. Kennedy.

Trump picked up on the story on Tuesday, stating on Fox that Cruz's father was with Lee Harvey Oswald prior to Oswald being shot. "I mean, the whole thing is ridiculous. What was he doing with Lee Harvey Oswald shortly before the death, before the shooting?" Trump asked.

Trump's insinuation that Cruz's father had something to do with Kennedy's assassination triggered a fiery retort from the Texas senator.

"Listen, Donald Trump is a serial philanderer and he boasts about it," Cruz said, directly raising Trump's marital history for the first time. "I want everyone to think about your teenage kids. The president of the United States talks about how great it is to commit adultery. How proud he is. Describes his battles with venereal disease as his own personal Vietnam."

Cruz also called Trump "utterly amoral, narcissistic, pathological liar and a bully."

As for the insinuation that his father was involved in assassinating Kennedy, Cruz said, "Let's be clear, this is nuts. This is not a reasonable position. This is just kooky."

Trump not only insinuated that Cruz's father had links with Oswald. He likewise criticised Rafael Cruz for having sought God's intervention and in his son's presidential race, slamming the pastor for using the pulpit to advance his political beliefs.

Speaking to fellow church members recently via a video conversation with the American Family Association of Indiana, Rafael Cruz said: "I implore, I exhort every member of the Body of Christ [church] to vote according to the Word of God and vote for the candidate that stands on the Word of God and on the Constitution of the United States of America. And I am convinced that man is my son, Ted Cruz. The alternative could be the destruction of America."

Trump said the call to godly arms was disgraceful.

"I think it's a disgrace that he's allowed to do it," he said on Fox News. "I think it's a disgrace that he's allowed to say it. You look at so many of the ministers that are backing me more so than they're backing Cruz and I'm winning the evangelical vote. It's disgraceful that his father can go out and do that."

Meanwhile, Ohio Gov. John Kasich is also set to drop out of the race, according to multiple reports shortly after he cancelled a campaign event in Virginia.

Kasich had earlier vowed that he would remain in the race despite Trump's overwhelming victory in Indiana.

The news about Kasich broke just hours after Cruz suspended his campaign followed by Priebus declaring Trump as the "presumptive nominee."