WASHINGTON — The Democratic Party is so alarmed by the prospect of competing against Fred Thompson, a Republican who portrays himself as a successor to President Reagan, that it is advising campaigners how to attack the actor and former Tennessee senator.

Mr. Thompson is not expected to announce his candidacy for the Republican nomination until next month, but Democratic strategists fear the combination of his conservatism, southern charm, and populist style could make him a hard man to beat in 2008.

Some polls have already placed him in the top two Republicans, underlining the weakness of his rivals.

The Democratic National Committee’s campaign focuses on what it regards as the fallacy in Mr. Thompson’s portrayal of himself as an outsider ready to shake up Washington.

According to the Politico newspaper, the committee has launched a pre-emptive strike, sending emails to members and branding Mr. Thompson “the inside-outsider.”

It points out that Mr. Thompson has enjoyed a decades-long career as a Washington lobbyist, and is the “ultimate Washington insider.” His acting roles came as a result of his political connections, beginning with small film parts and ending in a leading part on the television drama series “Law and Order.”

Despite casting himself as a plain-spoken everyman, Mr. Thompson has turned lobbying into a lucrative sideline. He earned more than $750,000 from a firm that wanted to limit its liability from asbestos lawsuits, and he helped to raise about $5 million for the legal fees of Vice President Cheney’s disgraced aide I. Lewis “Scooter” Libby.

“This is no political outsider,” said Craig Holman, a lobbyist for government ethics and campaign finance reform. “He clearly gained a network of contacts in Congress that he cashed in on and would represent anyone who would pay him.”