Mr. Christie, who is known to be sensitive to slights, appears to be genuinely stung by the network’s coverage. But his outrage is also fueled by opportunism, Republican strategists say: The same brand of politics that appealed to the liberal anchors on MSNBC has antagonized conservatives around the country, whose support he will need to mount a White House run.

Those close to Mr. Christie say the attack on MSNBC reflects his office’s effort to assert control over a story that has quickly snowballed, attracting new allegations almost every day, catching the governor off guard. Allies of Mr. Christie sense that Democrats are now unleashing the kind of assault on him that they failed to deliver during his re-election campaign last year, which he won — in a blue state — by a landslide.

“The Democrats have been making up for lost time,” said Kevin Madden, a former campaign strategist for Mitt Romney.

The rough patch in Mr. Christie’s relationship with the news media is especially striking given how assiduously he courts press attention. When Mr. Kornacki was a reporter in New Jersey, he recalled, he received personal emails from Mr. Christie, who was United States attorney at the time. Reporters have marveled at the access the governor has given them, including Jake Tapper of CNN, who declared that he had not experienced anything similar since John McCain’s famously freewheeling presidential campaign in 2000.