The Republican National Convention kicked off with fireworks Monday morning between the Donald Trump campaign and the governor of the host state, with campaign manager Paul Manafort calling former Trump rival Gov. John Kasich’s absence from the event “embarrassing.”

The Ohio governor has been an outspoken Trump critic, and has not buried the hatchet with the presumptive Republican nominee since suspending his own presidential campaign earlier this year. Manafort, who also is the convention manager for the presumptive Republican presidential nominee, told NBC’s “Today” that Kasich missing the convention “makes no sense.”

“He is embarrassing his state, frankly,” Manafort said.

Chief Kasich strategist John Weaver hit back at Manafort late Monday morning.

“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 New York 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.”

The barbs underscored the lingering tensions between Trump and certain wings of the party, as the campaign tries to use to weeklong convention to forge unity going into the general election campaign against Hillary Clinton.

Manafort said on MSNBC it was a “big mistake” for Kasich to stay home.

"Most of the Republicans who aren't coming are people who have been part of the past," Manafort said.

Kasich, a rival of Trump’s for the Republican nomination, has not endorsed his party’s likely standard bearer and has been critical of Trump.

"Why would I feel compelled to support someone whose positions I kind of fundamentally disagree with?" he told Fox News in June.

Kasich’s lone win during the Republican primary was his home state of Ohio.

Despite the absence of Kasich and a number of other prominent Republicans, including Jeb Bush and several former presidents and presidential nominees, the Trump campaign is eager to use the convention to bring disparate wings of the party together.

While Kasich is sitting out the convention, several former GOP rivals are set to speak.

They include former Texas Gov. Rick Perry on Monday, to be followed later in the week by Texas Sen. Ted Cruz, Florida Sen. Marco Rubio, New Jersey Gov. Chris Christie and retired neurosurgeon Ben Carson.

Perhaps the biggest headliner Monday, though, is Trump’s wife Melania.

Trump told Fox News that he plans to attend on Monday; sources confirmed to Fox News he plans to introduce his wife.

Fox News’ John Roberts and The Associated Press contributed to this report.