A Democratic fundraiser for Congressional candidates said some planned to distance themselves from Mr Obama and not attack Mr McCain.
"If people are voting for McCain it could help Republicans all the way down the ticket, even in a year when the Democrats should be sweeping all before us," said the fundraiser, a former Hillary Clinton supporter.
"There is a growing sense of doom among Democrats I have spoken to... People are going crazy, telling the campaign 'you've got to do something'."
Concern was greatest among first-term representatives who won seats in traditionally Republican districts in the landslide of 2006. "Several of them face a real fight to hold on to those seats," the fundraiser said.
Tony Podesta, a senior Democratic lobbyist, said members of Congress were "a little nervous" after Mr McCain shook up the race with his choice of Sarah Palin as running mate and intensified attacks on Mr Obama.
"Republicans have been on the offensive for the past two weeks... You don't win elections on the defensive."
For the longest time, all you ever heard was how the Republicans would be distancing themselves from President George W. Bush.
Sen. John McCain, though, had the President scheduled to appear at the Republican National Convention. That fell through when the President canceled a personal appearance when the hurricane struck the Gulf of Mexico. He still appeared via satellite.
Now, rather than the Republicans running away from President Bush, it seems the Democrats may be running away from Sen. Obama.
Like rats from a sinking ship.