MSNBC has not renewed its contract with Joan Walsh, a political analyst who had appeared on the network for years.

Ms. Walsh, who is also a national affairs correspondent for The Nation and was previously the editor in chief of Salon, lamented the decision on Saturday on Twitter, saying she had learned her fate the night before while baking with her daughter.

“I’ve given my heart and soul to the network, from the George W. Bush years through today,” she said. “I’m proud of the work I did.”

Ms. Walsh said in her tweet that she had worked with MSNBC for a dozen years, including six on contract. Network officials could not immediately confirm the length of her time as a paid contributor but issued a statement calling Ms. Walsh “a key voice on MSNBC for years.”

“Every year we review our paid contributors list across the ideological spectrum,” the statement said. “Unfortunately we couldn’t renew Joan, but she and her distinct perspective will still be invited on our shows.”

Officials said she would be welcome on MSNBC but as an unpaid contributor. Officials said the decision not to renew her contract was made based on the network’s budget and a desire to keep a diverse set of voices on the air.

She had appeared regularly on “Hardball” and “All In” on the network.

Later on Saturday night, Ms. Walsh said on Twitter that she would be joining CNN next year. “I am overwhelmed by the support I’ve received today from all of you,” she wrote. “Thanks to everyone who made this happen. A Christmas miracle.” A CNN spokeswoman confirmed late Saturday that Ms. Walsh would be joining the network as a commentator.

Reaction to MSNBC’s decision came swiftly. Among those upset was the longtime news anchor Katie Couric.

“Ugh. Love Listening to Joan,” she wrote on Twitter, adding the hashtag, “#KeepJoanWalsh.”

The network has leaned heavily on opinionated commentators — mostly facilitated by a stable of liberal anchors — and growing interest in the news to foster its rise. MSNBC this year surged into the top spot in prime-time cable news for the first time in 17 years.

Still, Andrew Lack, the chairman of NBC News since 2015, has insisted that the network’s mission is nonpartisan news gathering and has rejected the notion that MSNBC serves as a counterweight to the conservative Fox News.

“I don’t buy it,” he said in a June interview with The New York Times. “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.”