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IOWA - Actor-politician Fred Thompson opened an autumn scramble for the Republican presidential nomination on Thursday, pushing conservative themes espoused by the late Ronald Reagan to try to generate excitement in his long-delayed campaign.

Thompson, 65, hoped to offer a fresh face in a Republican field that has failed to energise many party members, but some believed he was entering the November 2008 race too late and formidable challenges loom.

"It doesn't matter what the pundits say today. We will win next year when it counts," Thompson told a crowd of about 200 voters who occasionally shouted "Go Fred Go" during a speech opening an initial campaign drive.

In a Fox News Channel interview, he said "I like the idea of a sprint" and shrugged off the concerns. "I'm right where I want to be, to tell you the truth."

Appearing with his wife, Jeri, Thompson stuck to broad themes of security, unity and prosperity in a 20-minute stump speech and said the United States faces a long struggle in the war against Islamic extremism of which Iraq is a part.

Attempting to catch candidates who have been canvassing Iowa for months, Thompson got off to a leisurely start, with his first campaign event taking place in mid-afternoon and a second one planned for later in Council Bluffs.

"I am determined that we make the decisions that will leave us a stronger nation, a more prosperous nation and a more united nation, and that's why I'm running for the presidency of the United States," Thompson said.

His deep rumbling voice, cultivated in a career in television and the movies, carried authority but his low-key delivery lacked a great deal of applause-getting rhetorical flourishes.

After "testing the waters" for six months and twice shaking up his staff, the tall, folksy Tennessean now faces an abbreviated time frame to try to increase his name recognition, raise money and overcome is Republican rivals by the time the first voters go to the polls in January.

Thompson's speech and Web video announcing his candidacy had many elements that had been the gospel of the hero of Republican conservatives, former President Reagan, such as a desire for low taxes and less government intervention.

And in a subtle jab at two other Republican candidates who are leading the polls but have been accused of flip-flopping on key issues, Rudy Giuliani and Mitt Romney, Thompson said he had held "the same common-sense conservative positions" throughout his political career.

Some analysts do not believe Thompson's seeming desire to wear the Reagan mantle will work.

Presidential scholar Stephen Hess of George Washington University said Reagan offered a far more optimistic presence than Thompson.

"I think the only thing they have in common is they were both actors, frankly," said Hess.

A senior Thompson adviser, Rich Galen, rejected any Reagan comparison. "He has consistently said there was only one Ronald Reagan and that he's not it," Galen said.

Iowa voters who attended the Thompson event said they did not mind his late entry - and in fact thought the '08 race had started too soon. One said he was concerned Thompson had waited too long.

"I'm afraid that's a possibility," said John Malett, 70, a retired school teacher. "I would have liked to have seen him enter a little earlier, but it's not insurmountable."

But other voters were not concerned. Scott Anderson, 51, said he liked Thompson's "down to earth sincerity" and Cheryl Ellis, 43, said she liked Thompson's view that the Iraq war must be won.

"He didn't just jump in," said Henry Hill, 23. "He stood back and thought about it."

Thompson drew fire from Republican rivals for choosing to appear on "The Tonight Show" instead of at a debate on Wednesday night sponsored by the New Hampshire Republican Party.