The pledge was originally made at the first GOP debate in August, when Trump held out a possibility of an independent run for president. With Trump now the front-runner, none of the remaining three candidates at the town hall would stick to previous statements they would support the eventual party nominee if it was not them.
Sen. Cruz, R-Texas, made it personal.
"I'm not in the habit of supporting someone who attacks my wife and my family," he said referencing comments Trump has made about Heidi Cruz. "Nominating Donald Trump would be an absolute trainwreck, I think it would hand the general election to Hillary Clinton."
Trump complained he has "been treated very unfairly" by the Republican Party establishment and said he wouldn't support the eventual nominee.
The battery charge against Trump's campaign manager Corey Lewandowski for grabbing a reporter was a key issue during the evening, with Cruz saying he would "of course" fire his manager for the same behavior.
Kasich told Cooper, "I haven't seen the video but they tell me the video is real and of course I would."
On foreign policy, Trump called NATO "obsolete," and appeared to be concerned about nuclear proliferation because it "is going to happen anyway."
On domestic policy, Trump said education, healthcare and security were the key functions of the federal government, though the idea clashed with long-held Republicans beliefs that the Department of Education should be abolished and the Affordable Care Act repealed.
Cooper also challenged Trump for being childish over jibes towards Cruz's wife, Heidi.
Kasich slammed Cruz and Trump for their policies towards Muslims in the United States. The governor quoted New York police Commissioner Bill Bratton's criticism of Cruz's plan as "ridiculous" and referenced the Easter Sunday terrorist attack in Pakistan and said "when people in Pakistan die, we all die a little bit."