(CNN) -- After months of not-so-coy will-he-or-won't-he political flirtation, Fred Thompson has finally and officially announced that he is a candidate for the 2008 Republican presidential nomination.
Fred Thompson joins the race for the GOP nomination only four months before the first voting.
"I am running for president of the United States," he said during a taping of NBC's "The Tonight Show with Jay Leno" Wednesday evening, drawing applause and cheers from the audience.
It's the same venue that helped launch the electoral career of another celebrity-turned-politician, California Gov. Arnold Schwarzenegger.
Thompson also talked about his decision to join the race in a webcast that went online early Thursday.
"I'm going to give this campaign all that I have to give, and I hope that you'll join me," Thompson said in the video announcement posted on his campaign Web site. "Our country needs us to win next year, and I'm ready to lead that effort."
Thompson, 65, who has had a multifaceted career as a prosecutor, lobbyist, actor and U.S. senator from Tennessee, is angling for a conservative base that has yet to coalesce around a favorite.
His decision to run was widely expected, after he formed an exploratory committee in June to begin raising money.
But political observers say Thompson may be late by entering the race months after his GOP rivals.
The first voting is only four months away and the other candidates have been in Iowa, New Hampshire, South Carolina and beyond for months.
But Thompson dismissed suggestions that he took too long to make the decision.
"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 told Leno. "If you can't get your message out in a few months, you're probably not ever going to get it out."
In his video announcement, Thompson touted his support for what he termed common-sense conservative principles, including low taxes, free markets, balanced budgets and the "sanctity of life."
"These principles made our country great, and we should re-dedicate ourselves to them, not abandon them," he said.
On the Iraq war, Thompson also challenged Democrats pushing for a withdrawal of U.S. forces, embracing President Bush's view that the U.S. effort in Iraq is a central front in the war on terror.
"Our courage as a people must match that of the brave men and women in uniform fighting for us," he said. "They know if we abandon our efforts, or appear weak and divided, we'll pay a heavy price for it in the future."
Even before he became a candidate, Thompson showed well in recent national poll averages, trailing only the front-runner, former New York Mayor Rudy Giuliani.
His acting role on NBC's popular "Law and Order" TV shows has buoyed his name recognition and prompted comparisons to another actor-turned-president beloved by conservatives, Ronald Reagan.
Thursday morning, Thompson will begin a two-day tour across Iowa, whose precinct caucuses traditionally kick off the nominating season in January. See Thompson's new presidential campaign buses »
He will travel from Des Moines to Council Bluffs, Sioux City, Mason City, Cedar Rapids and Davenport on a brown, yellow and red bus festooned with the slogan "United By Our Core Beliefs."
After his Iowa tour, Thompson will take his campaign to three other states with early primary contests -- New Hampshire, Florida and South Carolina.
"Expectations are sky high, which means he'd better perform flawlessly out of the box or a lot of people are going to start grumbling that he's not the great savior we thought he was," said Republican pollster Whit Ayres.
Thompson's virtual campaign kickoff came just hours after his GOP rivals held their latest campaign debate Wednesday night in New Hampshire -- an event he skipped to unveil his political plans on the set of "The Tonight Show."
Thompson joked that while he meant no disrespect to the organizers of Wednesday's Granite State event, "It's a lot more difficult to get on 'The Tonight Show' than it is to get into a presidential debate."
Over the summer, Thompson's nascent non-campaign campaign ran into turbulence with the departure of several staffers, including veteran Republican campaign adviser Tom Collamore, who left after disagreements with the candidate's wife, Jeri, over organization and staffing decisions, according to Republican sources familiar with Collamore's decision.
Jeri Thompson, 40, was a GOP media consultant before marrying Thompson in 2002.
The political world was also roundly unimpressed with the $3.4 million Thompson's exploratory committee raised during its first month of operation in June -- a fundraising pace far weaker than his GOP competitors.
Despite those bumps and his late start, Thompson has said he believes his campaign is still well-positioned to win the nomination.
Thompson's government service goes back to 1969, when he became a federal prosecutor in Nashville.
His profile went national in 1973, when he was appointed as the Republican counsel to the Senate Watergate Committee investigating misdeeds by President Richard Nixon and members of his staff.
During televised hearings that riveted the nation, it was Thompson who famously and dramatically asked White House aide Alexander Butterfield about the existence of tapes made of Oval Office conversations, which would eventually turn out to be Nixon's undoing.
After Watergate, Thompson returned to Tennessee to practice law. He fell into a career as an actor when he was asked to play himself in the 1985 movie "Marie," which was based on a real-life case in which he represented a whistleblower who exposed corruption. His performance in the film led to other film and TV roles.
In 1994, Thompson sought political office for the first time, running for the remaining two years of the Senate term that Democrat Al Gore gave up when he was elected vice president in 1992.
After barnstorming the Volunteer State in a red pickup truck while wearing cowboy boots, Thompson won in a landslide, and, in 1996, easily won re-election to a full six-year term.
In 2002, however, Thompson decided not to seek re-election and left the Senate. He worked as a lawyer-lobbyist in Washington and also took on the role of the gruff, conservative New York District Attorney Arthur Branch on "Law and Order."
Thompson left the show earlier this year when he began considering a presidential bid.
In April, Thompson disclosed that in 2004, he had been diagnosed with non-Hodgkin lymphoma, a cancer of the immune system. However, he said the cancer was in remission and he was suffering no symptoms. E-mail to a friend
CNN's John King, Bethany Swain and Doug Schantz contributed to this report.
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