DES MOINES, Iowa (CNN) -- As Fred Thompson officially hits the campaign trail, he's making a pitch to conservative Republicans nervous that the 2008 presidential race could lead to a Democratic president -- and even one named Clinton.
Former Sen. Fred Thompson makes his first campaign stop in Des Moines, Iowa, on Thursday.
Thompson says he's the candidate who could prevent that from happening by campaigning on true conservative values.
"To my Republican friends, I point out that in 1992, we were down after a Clinton victory," Thompson said in a video announcement posted to his campaign Web site Thursday morning.
"In '94, our conservative principles led us to a comeback and majority control of the Congress. Now, you don't want to have to come back from another Clinton victory. Our country needs us to win next year, and I'm ready to lead that effort," he said.
Thompson made his first official campaign appearance in Des Moines, Iowa, after finally announcing his candidacy for the 2008 Republican presidential nomination Wednesday evening on "The Tonight Show with Jay Leno." Watch what Thompson must do to gain political momentum »
"I still have the same common-sense conservative beliefs," Thompson said in Iowa, citing his belief in the "sanctity of life, lower taxes, less regulation, market economy, free competition, respect for private property rights, free and fair trade."
"If we stick with that, be successful and continue to be prosperous in this country," Americans will vote for the Republican ticket, he said.
Thompson's campaign launch didn't start without a hitch. As Thompson was preparing to officially launch his bid for the nomination, a senior aide resigned from the campaign.
Mark Corallo, a veteran of the Bush administration who was Thompson's primary spokesman when the Tennessee Republican began exploring a bid earlier this year, has left the campaign, Corallo and several Thompson aides told CNN.
Corallo declined to comment further on his departure. Two Thompson aides said Corallo and the campaign parted ways because of disagreements with the new campaign management team.
He is the fourth communications staffer to leave the campaign. Thompson also replaced his campaign manager weeks before Wednesday's announcement.
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 too late, as he entered 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."
Thompson's Republican rivals poked fun at his absence from a debate in New Hampshire on Wednesday night shortly before his announcement.
Arizona Sen. John McCain said, "I think that's a decision that Fred should make, and maybe we're up past his bed time."
Former Arkansas Gov. Mike Huckabee said, "I was scheduled to be on Jay Leno tonight, but I gave up my spot to somebody else because I'd rather be in New Hampshire with these fine people."
And Former Massachusetts Gov. Mitt Romney poked fun at Thompson's delayed entry into the presidential race.
"[The] only question I have: Why the hurry, why not take some more time off? ... Maybe January or February might be a better time to make a decision about getting into this race."
In his video announcement, Thompson 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 began 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.
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, Thompson 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's 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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