DURHAM, N.H. – After months of testing the waters, former senator Fred Thompson of Tennessee jumped into the race for the Republican presidential nomination on late-night television Wednesday, joining a wide-open contest that has no clear front-runner to lead a demoralized party into the next election.

As his eight rivals were gathering for their fifth debate of the year, Thompson used an appearance on NBC’s “Tonight Show With Jay Leno” to kick off his campaign. “I’m running for president of the United States,” Thompson told Leno during the show’s taping early Wednesday evening.

He was set to follow that up at midnight with a longer video on his campaign Web site outlining his reasons for running. Asked by Leno why he was skipping the New Hampshire debate, he said, “I’ll do my share, but I don’t think it’s a very enlightening forum, to tell you the truth.”

The well-choreographed Thompson rollout appeared designed to overshadow candidates who have been pounding the campaign trail for most of the year.

His long-awaited announcement brings a potentially formidable candidate into the Republican race. His Southern roots, conservative message and celebrity appeal from films and television’s “Law & Order” already have pushed him into second place in most national polls, behind former New York mayor Rudolph Giuliani.

But Thompson’s late start leaves him well behind his rivals in organizing his campaign in early states such as Iowa and New Hampshire, where former Massachusetts governor Mitt Romney has used television ads to build a lead in the polls, and in South Carolina and Florida, where Giuliani is currently ahead.

Thompson’s entry could quickly alter the dynamics of a Republican nominating battle that has evolved rapidly through the course of the year. When the campaign began, Arizona Sen. John McCain was seen as the likeliest candidate to claim front-runner status, but he ran into a series of problems over Iraq and immigration. Nearly broke at the end of June and losing ground in the polls, he jettisoned top staff and scaled back his campaign.

The summer belonged to Giuliani and Romney. Romney surged in polls in Iowa and New Hampshire and capped off August with a victory in the Iowa Republican straw poll – a contest Giuliani, McCain and Thompson all skipped. The victory solidified Romney as the favorite to win the Iowa caucuses and as a next-door neighbor and part-time resident of New Hampshire, he is also positioned well there.

For Giuliani, the summer helped overshadow the storyline that, despite his celebrity appeal, he had little chance of becoming the Republican nominee because of his support for abortion and gay rights. His support nationally has held up through a busy period of campaigning, establishing him as a credible, if still unconventional, threat to win the nomination.

The summer also helped to cement former Arkansas governor Mike Huckabee as the dark-horse candidate with the best chance of surprising one of his better-known rivals. His second-place finish in the Iowa straw poll gave his campaign a publicity boost as he staked his claim as the race’s true conservative.

Now will come another phase in the race, in which Thompson attempts to capitalize on the lack of enthusiasm among Republican voters for the presidential choices appearing on stage for the debate in New Hampshire last night.

But his start-up period proved extremely rocky. His early fundraising was viewed as a disappointment and he went through a succession of senior campaign advisers before recruiting Bill Lacy, who managed his 1994 Senate campaign.

Thompson advisers believe a clear and consistent message will be the key to success in the next few months. “Success for us will be if we are able to communicate Senator Thompson’s mainstream, conservative message to a Republican electorate that, frankly, until now, has been hungry for a new voice,” said communications director Todd Harris.

Wednesday’s debate was the fifth for the Republicans, whose last encounter was in August in Iowa.

Thompson’s campaign began airing his first campaign commercial Wednesday, a spot in which he warns that “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 concludes the ad with an invitation to visit his new campaign Web site, Fred08.com, on Thursday to watch a 15-minute video in which he officially announces his campaign.

The ad, titled “Debate,” was set to air throughout the day Wednesday and during the New Hampshire Republican presidential debate on Fox Wednesday night. A new and slightly modified version, titled “I’m in,” will replace the ad starting Thursday.

The idea, advisers say, is to create a national buzz about Thompson’s official entry into the race and drive traffic to the new Web site on Thursday. The 15-minute video went live at 12:01 a.m. Thursday morning.

Advisers hope it steals the show the next morning on talk shows and cable news networks. “The goal … is to drive as many people as we can to our Web site in anticipation of the launch video,” Harris said.

Thompson’s first attempt to communicate that message in more traditional ways will begin Thursday morning, when he will board a bus in Des Moines for a three-day trek around the perimeter of the state.

He will then spend two days in New Hampshire and a day in South Carolina.

Thompson’s rivals have already tried to attack his conservative credentials by questioning his commitment to the anti-abortion movement and his sponsorship of campaign finance reforms that are anathema to many in the party.

His advisers hope that they can cement the narrative about Thompson’s credentials before the attacks ramp up this fall. Said Harris: “There’s no question that we need to have a good announcement tour.

Best known for his portrayal of Arthur Branch, the gruff New York district attorney on “Law & Order, Thompson, 65, began his career in politics when he was recruited by then Tennessee senator Howard Baker to serve as the Republican counsel on the Senate select committee investigating the Watergate scandal.

An early case as a lawyer opened a door to Hollywood, where he has appeared in numerous movies over the years. He was elected to the Senate in the 1994 GOP landslide. He left the Senate in 2003, returning to Hollywood and to a lobbying career.

His first marriage ended in divorce. He later remarried and he has five children from the two marriages.

In April, Thompson revealed that he had been diagnosed with lymphoma in 2004 but said had been treated and is in remission.