Representative Bart Gordon, a 13-term Tennessee Democrat who is chairman of the House science committee, has never lost any of the 15 counties in his conservative district, let alone been in recent danger of losing his House seat.

He did not expect the outcome to be any different in 2010, though he acknowledged that members of Congress from both parties are going to have to work a little harder for re-election, given the sour national mood.

“I was going to be elected,” Mr. Gordon said. “I think that is pretty well understood.”

But his claim will not be tested. Mr. Gordon, 60, is leaving the House at the end of next year to explore other career opportunities, making him the 11th Democrat and the fourth centrist Blue Dog from a competitive swing district to announce that he will step down.

While it is true that a seasoned and popular congressman like Mr. Gordon probably could have been returned to Washington, it is hardly certain that the Democratic candidate to succeed him in a wide-open midterm race will have the same success, particularly in a district that Senator John McCain, Republican of Arizona, easily won in the 2008 presidential election.