Jim Bradshaw says that running for Congress as a defender of President Reagan's economic program makes him feel "a little like the sales manager for Tylenol."

"I do know my medicine works," says Bradshaw, the Republican candidate in the newly redrawn 26th Congressional District. "But all of a sudden, we've got a temporary problem with our product."

Sometimes Bradshaw just can't restrain himself. At a candidate forum here recently, he delivered a table-pounding condemnation of the Democrats who opposed the balanced budget amendment in the House, and when he was greeted with applause, leaned into the microphone and said sheepishly, "I just wanted to see if I had any supporters out there."

Two years ago, Jim Bradshaw was one of the best-known Republican challengers in America, and money flowed into his campaign treasury from across the nation. That's because he was trying to unseat House Majority Leader Jim Wright of Fort Worth.

He got clobbered. Asked what he is doing differently this time, Bradshaw replies, "Running in a different district."

Redistricting has given Bradshaw, a 42-year-old former Fort Worth city councilman, a second chance at Congress. He is running for one of three new seats awarded to the state because of its 27.1 percent population increase between 1970 and 1980.

The district includes the suburban cities between Dallas and Fort Worth, as well as two northern growth counties, making it a wonderful blend of old and new Texas. Along the highways, cattle ranches sit opposite hew housing developments, while elsewhere new office buildings coexist with courthouse squares strewn with ripening pecans.

Ronald Reagan is popular here, and the 26th was supposed to be Republican territory. But this race is in doubt because Bradshaw is running against a man many local politicians believe is the only Democrat who could carry such conservative territory.

He is Tom Vandergriff, 56, and for 26 years he served as mayor of Arlington, Tex., as it grew from less than 10,000 residents to more than 150,000. It is now the largest city in the district and Vandergriff campaigns on his role in this growth, including his help in bringing in an amusement park and the Texas Rangers baseball team.

Vandergriff retired in 1977, but his local ties are so extensive and his reputation still so solid that he has made this contest the best congressional race in the state.

Bradshaw has wrapped himself around the president. His campaign slogan is "Let's give President Reagan a hand, let's give him Jim Bradshaw." In contrast, Vandergriff is running as a local boy who brought jobs to the area as mayor and will do so again as a member of Congress.

"The issue is which of us is the most effective spokesman for the district," he says.

Although he favors a change of course in economic policy, Vandergriff calls himself a conservative and admires much about President Reagan.

"I would have stood with the president on many occasions in the last two years," he says. "But a phase of his plan has gone awry."

The polls show the race dead even, with a large percentage of voters undecided. That brings still one more line from the irrepressible Bradshaw.

"If it was anybody else who was my opponent," he says, "I could have gone to Europe and come back in January to get sworn in."