Mitt Romney. REUTERS/Jonathan Ernst Mitt Romney suggested on Monday that Manhattan billionaire Donald Trump would secure the GOP nomination if Texas Sen. Ted Cruz and Ohio Gov. John Kasich stay in the race until the July convention.

In a preview of his interview with David Gregory for his podcast, "The David Gregory Show," Romney insisted that stopping Trump depends on whether Cruz and Kasich continue campaigning.

"I think that Mr. Cruz and Mr. Kasich divide the vote, and that makes it easier for Mr. Trump to win the winner-take-all congressional districts and the winner-take-all states and get the delegates he needs to either hit the 1,237 or get close enough to sway the uncommitted delegates on the first ballot," said Romney, the 2012 GOP presidential nominee.

Romney added that if either drops out after Tuesday's New York primary, then he thinks Trump would not hit the needed 1,237 delegates to win the nomination. Instead, Republicans would enter into a contested convention.

He did not suggest who, between Cruz and Kasich, he believed should drop out of the race. Romney said last month that he was voting for Cruz in the Utah caucuses, which Cruz ended up winning, but said his encouragement was not an endorsement.

Romney made robo-calls for Kasich and campaigned for him ahead of the March 15 Ohio primary, which was the only contest Kasich won.

Since early March, Romney has been a leading figure in the anti-Trump movement, and the pair have engaged in a vicious back and forth that has cooled down only recently.

The former Massachusetts governor also opined on the rules the Republican Party has for the delegate-selection process — one that Trump has called "rigged" and "crooked" — as pro-Cruz delegates have been chosen across the country to spots that are committed to Trump on the initial ballot.

"Well, actually at this stage of course the rules are the rules, and the party put them out there well over a year ago," Romney said. "And they're very similar to the rules that existed at the time when I was running four years ago."

"By the way, there's probably nothing wrong with making it difficult," he continued. "After all, these rules are a lot simpler than the rules of foreign affairs, for instance, or the rules of our economy. And if you want to be president, you're going to have to deal with rules far more complicated than Republican delegate rules."

Listen to the preview of Gregory's interview with Romney below: