DES MOINES, Iowa — Fred Thompson — veteran actor, former Republican senator — is launching his bid for the presidency Hollywood style.

Mr. Thompson made his candidacy official in a 15-minute Web cast at midnight, around the same time that he was to be seen on the East Coast chatting with Jay Leno on a taped broadcast of NBC’s “Tonight Show.” He called attention to his bid hours earlier with a 30-second ad during the eight-man GOP debate in New Hampshire that he’s skipping.

“On the next president’s watch, our country will make decisions that will affect our lives and our families far into the future. We can’t allow ourselves to become a weaker, less prosperous and more divided nation,” Mr. Thompson says in the ad that will air on Fox News.

Mr. Thompson, 65, enters a crowded GOP field and an extraordinarily fluid race four months before the first votes. While Mayor Giuliani leads in national polls, Mitt Romney maintains an edge in the early voting states of Iowa and New Hampshire. Overall, Republican voters have expressed less satisfaction with their choices than Democrats, which Mr. Thompson sees as an opening for his candidacy. It won’t be easy for the former Tennessee senator. His campaign has been beset by lackluster fund raising and multiple staff changes, the most recent coming on Tuesday with the departure of his spokesman of just two weeks, Jim Mills. His made-for-television entry — and absence from the GOP debate — didn’t go over well with some in New Hampshire.

“There is a genuine interest in Senator Thompson here, a real curiosity about him,” New Hampshire’s Republican chairman, Fergus Cullen, said Tuesday. “But that curiosity is giving way to skepticism and maybe even cynicism about him in part because of how he’s handling his grand entrance. For him to then go on Jay Leno the same night and be trading jokes while other candidates are having a substantive discussion on issues is not going to be missed by New Hampshire voters.”

Mr. Thompson starts some eight months after his rivals began their own campaigns and lags behind Messrs. Giuliani and Romney in both money and organization. In June, Mr. Thompson fell short of his $5 million fund-raising goal by $1.5 million.

Mr. Thompson is perhaps best known district attorney Arthur Branch on NBC’s crime drama “Law & Order” and for his roles in more than a dozen movies.