Four years ago, John R. Kasich considered it unthinkable to vote for Donald J. Trump for president, yet he could not bring himself to cast a ballot for Hillary Clinton either — so he wrote in Senator John McCain of Arizona.
Not this time around. Mr. Kasich, a lifelong Republican who has spent decades in politics — as a state legislator, congressman, governor of Ohio and presidential aspirant — will become his party’s most prominent leader to cross the partisan divide and support former Vice President Joseph R. Biden Jr., speaking at this week’s Democratic National Convention, and then voting for him this fall.
“I wasn’t going to vote for somebody else,” Mr. Kasich said in an interview before his speech on Monday night. “I just wasn’t going to do it this time. I did it the last time. You know, I had always been hopeful, even after the convention and after the election, that perhaps we would see a change in the president, but we just never have. I happen to think it’s the soul of our country that is being damaged, and that’s what I’m concerned about.”
Mr. Kasich is an unlikely featured speaker at a Democratic convention. As a onetime brash young House Budget Committee chairman, he embarked with maniacal energy on a budget-cutting mission as part of Newt Gingrich’s Republican revolution in the 1990s and later clashed with labor unions as a governor allied with the conservative Tea Party in the 2010s. But he has moved away from the sharper edge of politics in recent years, focusing on issues of poverty and mental illness and even breaking with conservatives to expand Medicaid as part of President Barack Obama’s health care law.
He was the last man standing between Mr. Trump and the Republican nomination in 2016 and has spent the four years since then periodically speaking out against him, even as most of his Republican colleagues signed up for the president’s team or remained quiet.
Mr. Kasich insists that his opposition to the president is not driven by sour grapes.
“I don’t have any personal anger or anything toward him, I just don’t,” Mr. Kasich said. “It’s nothing to be taken personally. I just fundamentally disagree with the whole approach, and I’m deeply worried about our nation. I think if we continue this, I worry about how we ever will recover.”
On Monday, Mr. Trump, who had largely ignored Mr. Kasich’s defection, said of the former governor, “He was a loser as a Republican and he’ll be a loser as a Democrat. Major loser as a Republican. I guess you can quote me on that. John was a loser as a Republican. Never even came close. And as a Democrat he’ll be an even greater loser.”
Not all Democrats think he will fit in, or should. To some on the left, the party is abandoning its principles by showcasing a Republican whose positions on abortion, Social Security, labor and other issues have been at odds with Democratic orthodoxy. Among Democrats surveyed by CBS News, only 38 percent want to hear Mr. Kasich speak at the convention, compared with 72 percent for Senator Elizabeth Warren of Massachusetts and 63 percent for Representative Alexandria Ocasio-Cortez of New York, two liberal champions who will also get airtime.
“The party should be focused on energizing Democratic voters rather than using their convention to reassure billionaires, corporate donors and Republican lobbyists that they won’t actually try to challenge the status quo,” said David Sirota, a former speechwriter for Senator Bernie Sanders, independent of Vermont, who is also speaking Monday night.
Mr. Biden and his allies argue that Democrats should welcome anyone to their battle to remove Mr. Trump from the White House, and that a purity test would be self-destructive.
“It’s great to see Kasich as a speaker at our convention,” said former Gov. James J. Blanchard of Michigan, a Biden delegate and a Democrat, because Mr. Kasich “represents that group of independents and moderate Republicans” Mr. Biden is seeking to win over.
How much Mr. Kasich can attract other disaffected Republicans to support the Biden campaign is up for debate. He opted against running against Mr. Trump for the Republican nomination this time around, a concession to the president’s command over the party. “Couldn’t win. It’s a nonstarter,” he said of such a challenge. “There was no way to get traction.”
Mr. Kasich said he remains a Republican and acknowledged that he has deep disagreements with Mr. Biden on issues, but said that individual policy differences are less important than the broader crisis confronting the country.
“Somebody has to start being willing to break down this tribalism,” he said. “I don’t think that I will achieve that, but somebody’s got to start. Somebody has to be able to begin to say, ‘No, you’ve got to take off your partisan hat, and you’ve got to realize we’re Americans.’”
“With Biden, who I’ve known for a long time, I just think that he has the ability to be patient and bring people together and to try to solve some of our big problems by engaging in dialogue,” he added. “Nothing great was ever achieved in this country where we’re fighting with each other like we are now, nothing. And significant things can be accomplished when there’s respect on all sides regardless of some fundamental differences.”
Michael Shear contributed reporting.