CLEVELAND — Walk around the Republican convention and talk to Ohioans, to Republicans from around the country, and to party strategists about the feud between Donald Trump and John Kasich, and here's the short version of what you'll hear: Kasich is being a jerk, but Trump is crazy to fight with him.

The Trump-Kasich spat is more than a sideshow. It's at the very heart of the presidential campaign. Of the various ways, none of them easy, for Trump to win the White House, the most direct is: Win all the states Mitt Romney won in 2012, and then, on top of that, win Ohio, Pennsylvania and Florida. Any other path starts to get really complicated. So a Republican who loses Ohio loses the presidency.

This fall, and especially in October, the GOP presidential candidate will need a huge assist from the Republican power structure in Ohio. He simply has to have it. And at the moment it looks very much like that won't happen.

"The governor has said that the nominee has to have a positive, inclusive vision, and unless there is a Saul-on-the-road-to-Damascus conversion, he is not going to support [Trump]," Kasich spokesman Chris Schrimpf told me in a phone conversation Tuesday. "Whatever happens in Ohio for Donald Trump is a choice that Donald Trump will make, based on how he chooses to campaign."

The feud started as bad blood during the primary campaign. It continued after Kasich dropped out and refused to support Trump. And now, Kasich has made history by boycotting his own party's convention in his own state. Finally, on Monday, came the shots from Team Trump.

"He's embarrassing his party in Ohio," Trump campaign chief Paul Manafort said of Kasich during at a breakfast meeting, reported by the New York Times. "Negotiations broke down because John Weaver thinks that John Kasich will have a better chance to be president by not supporting Donald Trump."

Weaver, once a Republican, and then a Democrat, and now a Republican again, is Kasich's campaign guru, widely thought to be encouraging Kasich to run for president again in 2020. After Manafort slammed Weaver, Weaver hit back, zeroing in on Manafort's lucrative international lobbying career. "Manafort's problem, after all those years on the lam with thugs and autocrats, is that he can't recognize principle and integrity," Weaver told the Times. "I do congratulate him though on a great pivot at the start of the convention after such a successful vice-presidential launch. He has brought great professionalism, direct from Kiev, to Trump world."

Weaver, whose candidate won 37 fewer states and 9,135,191 fewer votes than Trump in the primaries, also called the Trump campaign a "clown show," run by "knuckleheads."

Trump himself shot back. Calling in to Fox News on Monday night, Trump said of Kasich, "I beat him very, very soundly. And you have to understand, this was a contentious, some people say the most contentious primary they have ever seen in either party. If I were him and gotten beaten that badly I probably wouldn't show up either. He has a problem that he signed the pledge. And from a standpoint of honor I think he should show up."

Team Kasich maintains the governor himself is not trying to cause a conflict. "He is keeping a low profile," Schrimpf told me, "trying to be a good host, trying to be polite, trying to stick to his principles. It's not us who said anything this entire week."

Weaver's remarks, Schrimpf said, were "a mild response to what the Trump people have done."

Whoever is really to blame, what makes Trump's handling of the Kasich problem so ill-advised is the fact that Kasich is truly popular here in Ohio. You know those moderate women suburban voters Republicans always wish they had more of? Kasich has them.

Yet at the same time, there are hints here at the convention that Kasich's supporters, many of whom voted for him in the March 15 Ohio primary, aren't happy with Kasich's decision to boycott the convention. Before the convention started, I received a note from a source who talked of unhappiness in the Ohio delegation ranks.

"They don't take Kasich's opposition to Trump as principle, but rather petulance," the source wrote. "One older man told me that whenever it's his guy who loses — a Tea Party candidate or a more conservative primary challenger — he's always been told that after the primary you have to fall in line behind the nominee, even if you disagree with him. Now, the same people who said that are telling him that they can't vote for Trump out of principle."

At a rally Monday for Ohio Republican Sen. Rob Portman on the campus of Cuyahoga Community College, some Kasich voters said much the same thing. Several told me Trump wasn't their first, or their second, choice, but now that he is the nominee, they're going to support him. They believe Kasich should, too.

"He signed the pledge," said one, referring to Kasich's promise to support the choice of GOP voters. "He ought to live up to it."

"I think [Kasich] should be gracious," said another. "Instead, he's being a little bit petty."

"For years we were told to get behind the nominee," said a third, "to work hard and get behind McCain, to get behind Romney. And we did. So now, we're being told we don't have to."

Kasich is famous for having a difficult personality. But as far as Trump is concerned, Kasich is the governor of a swing state Trump cannot afford to lose. Kasich controls political resources in that critical swing state. So it really doesn't matter that many see Kasich as a sore loser after the GOP primaries. Winners have to manage losers wisely. Or, at a minimum, not attack them publicly.

"The best way to handle that is, you're the victor — be gracious," said GOP strategist Karl Rove in a conversation Tuesday. "And when [you're] asked about it, say, 'Well, we're disappointed, because the delegates would love to hear from the host state governor… We're confident he would be warmly received at the convention. But that's his decision, and we understand that, and we respect that, and we've adjusted the schedule accordingly.' And move on."

Rove thought back to the 2000 Republican presidential primary race that ended with a lot of hard feelings among John McCain supporters. The George W. Bush campaign had to try to bring those supporters into the fold. Not attacking McCain was a start.

"[Kasich] has got feelings hurt because Trump has said ugly things about him," Rove said. "People are people, and it's sort of hard to put some of that behind you, and I guess that's why he's not [attending the convention]. But if I were him, I'd come [to the convention] and say what I want to say, which is welcome to my great state, where we're doing so many magnificent things."

Rove suggested that Kasich's ultimate stance toward Trump will depend on whether the governor's "self-interest aligns" with Trump's by this fall. But given the number of people who see Kasich's maneuvers as positioning for 2020 — "He's running," one Ohio strategist declared flatly in a conversation Tuesday — it is difficult to see how that alignment might occur. Any chance for the 64-year-old Kasich to run again depends on Trump losing in November.

Insiders in both parties believe Trump has a chance to win Ohio. "He's going to kill it" in some key blue-collar areas of the state, the strategist told me. Some of Trump's weaknesses, like his low standing among Hispanic voters, won't be a big factor in Ohio. And dislike of Hillary Clinton is strong; the millions of dollars in ads she has aired in Ohio haven't made a huge difference, at least so far. It's probably fair to say that few would be surprised if Trump does better than Romney, who lost Ohio by three points in 2012.

But Trump has to do better than just out-perform Romney. He has to win Ohio, or lose the presidency. Which makes Trump's feud with Ohio's Republican governor more than just a quarrel left over from some tough primaries. It is a dispute that could determine the outcome of the entire national race.