Donald Trump. Joe Raedle/Getty Images

Sen. Ted Cruz of Texas may come to regret calling Indiana an "absolutely pivotal" state.

Republican presidential frontrunner Donald Trump is hoping to score a knockout punch against Cruz with a major victory in the Indiana primary on Tuesday.

Appearing at several campaign rallies across the state on Monday, Trump relentlessly doubled down on his personal attacks on Cruz in an attempt to bury him once and for all and virtually lock down the GOP race.

"I think he's crazy — honestly, I think he's crazy," Trump said at a rally Monday night.

He went on to question Cruz's "temperament," a line of criticism often aimed at Trump.

"Ted does not have the temperament to be doing this," Trump said. "He's choking like a dog because he's losing so badly. We've got to put him away tomorrow, folks."

Cruz has campaigned vigorously in the state, which was seen as more favorable to his candidacy.

The senator spent last Tuesday night in Indiana, a signal that he was focused on the importance of the Hoosier State contest rather than the five states he lost to Trump that evening. Cruz also chose Indiana as the location to announce his potential vice presidential running mate, former Hewlett Packard CEO Carly Fiorina. Cruz made five campaign stops Monday while his wife, Heidi Cruz, along with other high-profile surrogates appeared at five separate stops without him.

But the Cruz campaign has encountered numerous late setbacks in the Hoosier State. Recent surveys have shown Trump with a sizable lead over the Texas senator.

An NBC/Wall Street Journal poll released Sunday found Trump with a 15-point lead over the senator among likely Indiana Republican primary voters.

Cruz's plan to consolidate anti-Trump support also fell through.

Last week, Cruz attempted to forge an alliance with Gov. John Kasich of Ohio to persuade the governor's supporters in the state to vote for Cruz and block Trump. But the pact quickly fell apart.

Trump brimmed with confidence during a "Fox News Sunday" interview when asked whether Indiana would essentially end the Republican race.

"Yes, it's over," Trump said. "It's already over."