Mr. Olbermann may be long gone. Yet the network’s rise has relied largely on its sharply opinionated commentary, with Ms. Maddow moving to No. 1 in prime time on the strength of monologues devoted to President Trump’s ties to Russia, and with Lawrence O’Donnell speaking openly about impeachment.

In an interview, Mr. Lack rejected the notion that MSNBC was a Fox News for the left.

“I don’t buy it,” he said, his voice rising. “And honestly, I’ve never been comfortable with the Fox examples, of how we compare to them, or being an alternative to them. I don’t think we’re an alternative to anything. We’re live, breaking news during the day, and the smartest, most insightful opinion space we can create at night.”

The network’s marketing campaigns have promoted its reporting heft, with experienced journalists like Andrea Mitchell and Chuck Todd taking over daytime hosting duties from the likes of Al Sharpton. Mr. Lack, 70, who helped found MSNBC in 1996, said the investment in straight-news reporting had allowed the channel to compete during last year’s presidential race and led viewers to feel more comfortable turning to MSNBC during breaking news.

Without that journalism, he added, “I don’t think you have the success that you’re seeing.”

MSNBC’s morning and afternoon audience is growing fast: Its total viewership between 9 a.m. and 5 p.m. nearly doubled in May from the year before, far outstripping the growth at CNN and Fox News.

Over all, though, the network is still No. 3 in daytime. Its landmark ratings win in May came squarely in the liberal precincts of prime time, and there are signs the network is doubling down in that arena.