Fortier said the best message the Democrats can come up with is to blame former President George W. Bush and the GOP for the economic woes.

"These are not the best messages, but they are the best Democrats have," he said.

Some experts said it is too late for a complete about-face of U.S. voter sentiment against the majority Democratic Congress.

Many have already made up their minds on the elections, as people' s impressions of the economy are formed several months before the election and are not swayed by last minute good news, Fortier said.

Is there still a chance?

Fortier said the best Obama could do is to continue to help raising money for candidates in close races. In such races, some last minute deciders could make a difference.

Around 15 percent to 20 percent of voters might be expected to make up their minds in the last two weeks, he said.

For Obama, the best tactic would be to get out the vote in key districts, especially by galvanizing young people and African American voters the way he did during the presidential elections, Mahaffee said.

History on GOP side

History is also on the side of Republicans, as the party in power usually loses the first mid-terms after the election of a new president.

In both 1994 and 2006 -- the two most recent mid-term elections that saw a shift in the balance of power -- the percentage of voters saying most members of Congress should be re-elected fell below 40 percent, as it does today, according to Gallup, a polling organization.

While Republicans are not popular, either, they may still be able to coast into victory on a wave of anti-majority sentiments, experts say.