WASHINGTON (AP) — Former Ohio Gov. John Kasich, who ran against President Donald Trump in the 2016 Republican primary, said Friday that he supports impeaching the president — but isn't ready to call for his removal from office.

Kasich said he decided to back impeachment after hearing acting White House chief of staff Mick Mulvaney acknowledge Thursday that Trump's decision to hold up military aid to Ukraine was linked to his demand that Ukraine investigate the Democratic National Committee and the 2016 U.S. presidential campaign. Mulvaney later claimed his remarks were misconstrued.

"This is an extremely serious matter," Kasich told The Associated Press in an interview. "I wrestled with it for a very long time."

It marked a reversal for Kasich, who previously said he hadn't seen evidence of a quid pro quo on Trump's part. Congress is conducting an impeachment inquiry sparked by a whistleblower's allegation that Trump pressed Ukrainian President Volodymyr Zelenskiy to dig up dirt on Trump's potential 2020 Democratic presidential rival Joe Biden as Trump delayed military aid to the country.

"I can understand why executives would withhold military aid if it was in the sense of public policy, but you don't withhold military aid to anybody ... for political reasons," Kasich said.

He first announced his change of heart on CNN on Friday afternoon, calling Mulvaney's acknowledgement the "final straw" and saying, "The last 24 hours has really forced me to review all of this."

In an interview later with the AP, he declined to call for Trump's ouster, saying only, "Slow down, one thing at a time." Kasich said he wanted to see the articles of impeachment and the process by which the Democrats conduct their investigation.

"There's a long way to go and a lot of witnesses to see," he said.

Acknowledging that he's the rare Republican to have broken with Trump, Kasich said it was "difficult to believe (Republicans) think they can look the other way on these things." But he also declined to criticize his party, saying, "I'm not interested in berating them. I don't think it helps."

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Kasich is touring the nation promoting his new book "It's Up to Us," which he says outlines ways in which individuals have "the power to bring about change." The former Ohio governor said that he hasn't yet ruled out a 2020 bid for president, and that he still gets requests from supporters daily asking him to run for president. He emerged as the last moderate Republican standing against Trump during the 2016 presidential primary and bowed out of the primary after winning only his home state.

But he wasn't bullish about his chances this time around, lamenting that "there's no way for me to win right now." He noted that Republicans "are still very much behind Trump" and that some states have already canceled their Republican presidential primaries.

Kasich doesn't know who he'll vote for in 2020 — and he declined to answer when asked whether he'd support a Democrat. He is certain of one thing, though.

"I'm not going to vote for him!" he said.