Three Republican candidates are battling for their party’s nomination on Aug. 7 to face incumbent Rep. Lincoln Davis, D-Tenn., in the Nov. 4 general election.

Mr. Davis is seeking a fourth term in the 4th Congressional District, which includes Bledsoe, Franklin, Grundy and Marion among its 24 counties.

Dr. Bruce Oppenheimer, a politic science teacher at Vanderbilt University, said it would be a challenge for anyone to unseat Mr. Davis.

He said one problem is that the area is so large, stretching from the Kentucky border to the Alabama border before making a dogleg toward Nashville. The district touches three major media markets, which means challengers must raise lots of money to compete, he said.

“Davis has had a few terms now to solidify the district,” he said. “It’s a very conservative district and he’s a very conservative Democrat.”

Campaign officials for Mr. Davis declined to comment for this story.

But all the challengers said they think Mr. Davis could be vulnerable in the general election and claimed he aligned himself with House Speaker Nancy Pelosi and New York Sen. Hillary Clinton.

The candidates are Kent Greenough, a business consultant in Petersburg, Tenn.; Monty Lankford, a Franklin, Tenn., businessman, and Don Strong, a respiratory therapist from Crossville, Tenn.

Mr. Greenough and Mr. Strong cite military experience as a strength in their campaigns. Mr. Greenough and Mr. Lankford also cite business experience as strengths.

Mr. Greenough said six years as a Marine Corps officer on active duty and in the reserves makes him strongest on national security.

“It’s not just a feel-good, ‘I support the troops (attitude),’” Mr. Greenough said. “I was the troops.”

He said he has raised between $15,000 and $18,000. He did not raise any money during the first quarter, federal records show. He said he will focus on a grassroots campaign rather than trying to develop a war chest.

Mr. Lankford raised more than $200,000 in the first quarter. He said he thinks he’ll be able to connect with the conservative voters in the district.

“This is the only district where there’s a chance to oust the incumbent,” he said. “It’s getting more and more conservative.”

He said he would “educate” voters on Mr. Davis’ record, which includes voting against a bill that would have protected telephone companies from being sued for giving personal information to the U.S. government.

Mr. Strong said he has not yet raised any money and has self-financed his campaign. He said he does not think his values are that far apart from those of the other candidates.

Mr. Strong said he believes in less government and less spending.

“I don’t know if there’s a lot that separates us,” he said. “But folks know that I care.”

Dr. Oppenheimer said he thinks the candidates might be positioning themselves for a later run.

“What they might be doing is establishing themselves as candidates in 2010 if Davis plans to run for governor,” he said.