"There's no doubt there are enough seats in play that could cause Republicans to gain control, there is no doubt about that," Gibbs said on NBC's "Meet the Press."
Democratic candidates' success will hinge large on strong campaigns, he said.
Gibbs took aim at Republicans, particularly what he called the legacy of President George W. Bush's administration.
"We know the policies that led us into a financial calamity, massive budget deficits," Gibbs said.
He said the U.S. economy lost 3 million jobs during the final six months of 2008. By contrast, he said, 600,000 private-sector jobs have been created during the first six months of 2010.
The fall elections present a clear choice, he said: "Do you want to go backward to an economy that led us into this mess that saw the greatest financial calamity since the Great Depression, that turned a record surpluses into record deficits? Or do you want to continue on the track that the president has put us on that has create private-sector jobs?"
Gibbs acknowledged frustration over persistently high unemployment but noted the economy had been shedding hundreds thousands of jobs monthly until recently and said it showed signs of rebound.
"I'm not here to unfurl the 'mission accomplished' banner," Gibbs said. "We've got a lot of work to do."