ALBANY - Former New York Gov. George Pataki said he won't vote for Donald Trump for president even if he is the Republican nominee, touching off another Twitter skirmish between the two GOP presidential candidates.

"Let me be very plain, I'm not going to vote for [Trump]. He is unfit to be president," Pataki said on Twitter. He urged his Republican colleagues to take the same stand.

Trump, who regularly uses Twitter to shoot back at critics, didn't wait long to respond. He invoked his front-runner status to try to belittle Pataki.

Why is someone like George Pataki, who did a terrible job as governor of N.Y. and registers zero in the polls, allowed on the debate stage?" Trump wrote.

Which prompted rejoinder from Pataki:

"Why is a serially bankrupt @RealDonaldTrump who insults Mexicans, women, blacks, Jews & POWs on #2016 debate stage?" Pataki wrote.

Pataki perhaps has been the Republican who is most critical of Trump, sparring on Twitter and calling the developer's remarks about illegal immigrants "unacceptable." Some see it as Pataki's strategy to raise attention for his campaign.

The exchange came as Republican candidates are preparing to meet in Simi Valley, Calif., for the party's second presidential primary debate. It will be televised on CNN and its sister stations.