NEW YORK — MSNBC is replacing Keith Olbermann and Chris Matthews as co-anchors of political night coverage with David Gregory, and will use the two newsmen as commentators.

The change reflects tensions between the freewheeling, opinionated MSNBC and the impartial newsgatherers at NBC News. Throughout the primaries and summer, MSNBC argued that Olbermann and Matthews could serve as dispassionate anchors on political news nights and that viewers would accept them in that role, but things fell apart during the conventions.

Gregory, the veteran Washington hand, will anchor MSNBC's coverage of the presidential and vice presidential debates and election night, network spokesman Jeremy Gaines said Sunday. The change was first reported by The New York Times.

The tipping point appears to have come during the GOP convention when Olbermann criticized MSNBC for showing a Sept. 11-themed video prepared by the Republicans.

MSNBC executives, who had publicly defended their anchors' roles while privately monitoring them throughout the political season, made the change over the weekend after discussions with Olbermann. Despite the controversy around him, Olbermann has been a hero with left-leaning viewers and keyed MSNBC's growth among coveted young viewers.

During her acceptance speech last week, Republican vice presidential candidate Sarah Palin talked about the "Washington elite" not accepting her qualifications for the job. Some delegates on the convention floor began chanting, "N-B-C, N-B-C."

Olbermann began to have difficulty keeping his opinions in check, or simply stopped trying.

He sarcastically dismissed GOP pundit Pat Buchanan on the air after Buchanan said the Republicans had been enlivened by the entrance of a conservative Republican.

"Those reading US Weekly with the picture of her and her youngest daughter with the word `scandal' written across it won't be so happy," Olbermann said.

He expressed little sympathy at another point when GOP anger at rumors over the Internet about Palin were being discussed.

"We'll see if people feel sorry for unfounded rumors on the Internet," he said. "If that's the case, Senator Obama's probably standing up and cheering and waiting for people to feel sorry for him."

Perhaps most embarrassing, Joe Scarborough was discussing positive developments in John McCain's campaign at one point when Olbermann was heard on an offstage microphone saying: "Jesus, Joe, why don't you get a shovel?"

Scarborough, a former Republican congressman and host of MSNBC's "Morning Joe," got in another nasty on-air exchange with MSNBC reporter David Shuster, and Matthews snapped at Olbermann on-air when it appeared Olbermann was criticizing him for talking too much.

All the drama made MSNBC a punch line when top NBC anchor Brian Williams appeared on Comedy Central's "The Daily Show" last week. "Is there no control?" host Jon Stewart asked him. "`Is it `Lord of the Flies?"'

A sheepish Williams said that every family has a dynamic of its own.

"But does MSNBC have to be the Lohans?" Stewart said.

Olbermann was in Denver during the Democratic national convention, but performed his co-hosting duties for the GOP convention in a New York studio. NBC President Steve Capus said the decision was not political, that Olbermann had been sent back to anchor coverage of Hurricane Gustav.

MSNBC's decision comes just before Olbermann's "Countdown" show is set to air, on Monday, his interview with Barack Obama. That will put Olbermann in direct competition with his nemesis, Fox News Channel's Bill O'Reilly, who interviewed Obama last week and is airing a portion of it Monday in the same 8 p.m. EDT time slot.