Fred Thompson - veteran actor, former Republican senator - launched his bid for the US presidency in Hollywood style. "I'm running for president of the United States," Thompson told Jay Leno in a taped appearance on NBC's "Tonight Show" airing Wednesday night. Thompson called top opponents Rudy Giuliani and Mitt Romney formidable but added, "I think I will be, too" as he rejected the notion that he was getting into the race too late, only four months before the state-by-state voting to narrow down candidates begins. The national election is in November 2008. "I don't think people are going to say, 'You know, that guy would make a very good president but he just didn't get in soon enough,"' Thompson said as the studio audience laughed. Poking at his rivals who have been running since the year began, he added: "If you can't get your message out in a few months, you're probably not ever going to get it out." Late-night talk shows have become a popular place for politicians to announce they are candidates for public office. Earlier this year, McCain announced his presidential bid on CBS' "Late Show With David Letterman." Arnold Schwarzenegger disclosed to Leno his plans to run for California governor. Thompson also is calling attention to his candidacy with a 30-second ad broadcast during a Republican debate in New Hampshire on Wednesday, a debate he skipped. He also is explaining the rationale for his candidacy during a 15-minute Webcast on his campaign Internet site just after midnight. "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," Thompson says in the ad that will air on Fox News Channel. Thompson, 65, enters a crowded Republican field. While Giuliani leads in national polls, 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 Thompson sees as an opening for his candidacy. It will not be easy for the former senator. His campaign has been beset by lackluster fundraising and multiple staff changes, the most recent coming Tuesday with the departure of his spokesman of just two weeks, Jim Mills. His made-for-television entry - and absence from the Republican debate - did not go over well with some in New Hampshire. "There is a genuine interest in Senator Thompson here, a real curiosity about him," New Hampshire 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." Several of Thompson's rivals warned Wednesday night that the nomination must be earned through hard work. "The only question I have for Senator Thompson is, Why the hurry? Why not take some more time off? Maybe January, February might be a better time to make a final decision about getting in this race," Romney quipped. On a serious note, Giuliani said: "This is not a time that the United States should be electing someone who's gonna get on-the-job training. You need people with executive experience." Thompson lags behind Giuliani and Romney in both money and organization. In June, Thompson fell short of his $5 million fundraising goal by $1.5 million. Still, Thompson consistently ranks among the top Republicans in national polls and state surveys. With a mostly right-leaning Senate record and a plainspoken style, he is looking to capitalize on discontent with the current choices among conservatives who make up a significant segment of Republican voters. Thompson is perhaps best known to millions of Americans as the gruff district attorney Arthur Branch on NBC's crime drama "Law & Order," as well as for his roles in more than a dozen movies. During his 1994-2002 Senate tenure, he was considered a reliably conservative vote. However, he strayed from the party line on a few issues, including advocating for campaign finance reform. He also was John McCain's campaign co-chairman in 2000 instead of backing establishment candidate George W. Bush. Thompson spent many years in Washington as a lawyer and a lobbyist. He has faced repeated questions about his lobbying work for a family planning group seeking to relax an abortion rule and former leftist Haitian leader Jean Bertrand-Aristide.