Similarly, on big Democratic primary nights  which tended to be clustered in the first three months of the year  CNN routinely outdrew Fox News, with MSNBC sometimes coming in second. Over the last three months, as Senator Barack Obama secured his hold on the Democratic nomination, viewership on all three cable news channels fell, with CNN losing the most viewers (about 35 percent) when compared with the first three months of this year, according to Nielsen estimates.

“I would think Democratic viewers would be more inclined to go to CNN,” said Carl Forti, executive vice president for issue advocacy for Freedom’s Watch, a conservative organization. “At the same time, there’s been no Republican primary since Feb. 5, and there hasn’t been a lot going on on the Republican side to drive Republicans to political coverage.”

But disproportionate interest in the Democratic campaign alone cannot explain the struggles of Fox relative to years past, and the gains of its competitors. CNN and MSNBC have somehow managed to photocopy several pages from the playbook of Roger Ailes, the founder of Fox News and its chairman, whose emphasis on sharp opinions, glitzy graphics and big personalities has been taken to heart by competitors like CNN’s Anderson Cooper, Mr. Olbermann and his running mate on MSNBC, Chris Matthews.

“What Ailes would say is, they’re following our lead,” said Bob Kerrey, the president of the New School in Manhattan and a former Democratic senator from Nebraska. “There is a certain element of truth to that.”

But, Mr. Kerrey argued, by making themselves more compelling and entertaining to watch, CNN and MSNBC put themselves in a far better position than Fox News to capture those young voters who were paying attention to a presidential campaign, particularly the race for the Democratic nomination, for the first time.

Meanwhile, Fox’s hosts have seemed to struggle on camera to find a new voice. Three times in less than three weeks in late spring, Fox acknowledged making inappropriate references to Mr. Obama, the presumptive Democratic nominee. It said on June 12 that it should not have referred in an on-screen headline to Mr. Obama’s wife, Michelle, as his “baby mama.” Previously, a Fox anchor, E. D. Hill, apologized for likening a seemingly affectionate fist bump by the Obamas at a rally to “a terrorist fist jab,” and a Fox analyst, Liz Trotta, expressed contrition for making a joke about a possible assassination of Mr. Obama.