The Keystone State's 71 delegates make it the biggest prize of the Tuesday slate.
Ted Cruz has bested Trump in winning delegates at past state conventions.
“I have never been more worried about the Republican Party breaking apart than I am today.”
The governor said the Cruz camp backed out of a deal for a "unity slate" of delegates.
"Every single debate, I beat Ted Cruz. He was supposed to be a good debater."
He linked Obama's foreign policy to Trump, pledging to "stand unapologetically" with Israel.
He said a contested convention would be the worst option for the Republican Party.
Trump had not brought up his questions about Cruz's eligibility in several weeks.
"People are starting to say, ‘wait a minute, look what got you here.’ ”
“Tough, strong, smart. I would like to get that for Indiana, I’ll tell you what."