NEW YORK — Despite sweeping Democratic successes in the past two national elections, continuing job losses and President Barack Obama's slipping support could lead to double-digit losses for the party in next year's congressional races and may even threaten their House control.

Fifty-four new Democrats were swept into the House in 2006 and 2008, helping the party claim a decisive majority as voters soured on a Republican president and embraced Obama's message of hope and change. Many of the new Democrats are in districts carried by Republican John McCain in last year's presidential contest; others are in traditional swing districts that have proved tough for either party to hold.

From New Hampshire to Nevada, House Democrats also will be forced to defend votes on Obama's $787 billion economic recovery package and on energy legislation viewed by many as a job killer in an already weak economy.

Add to that the absence of Obama from the top of the ticket, which could reduce turnout among blacks, liberals and young people, and the likelihood of a highly motivated GOP base confused by the president's proposed health care plan and angry at what they consider reckless spending and high debt.

Taken together, it could be the most toxic environment for Democrats since 1994, when the party lost 34 House incumbents and 54 seats altogether. Democrats currently have a 256-178 edge in the House, with one vacancy. Republicans would have to pick up 40 seats to regain control.

"When you have big sweeps as Democrats did in 2006 and 2008, inevitably some weak candidates get elected. And when the environment gets even moderately challenging, a number of them are going to lose," said Jack Pitney, a political science professor at Claremont McKenna College in California.

Since the mid-19th century, the party that controls the White House has lost seats in virtually every midterm election. The exceptions were in 1934, when President Franklin D. Roosevelt navigated the Great Depression, and in 2002, after the attacks of Sept. 11, 2001, strengthened George W. Bush's image as a leader.

With history as a guide, Rep. Chris Van Hollen, D-Md., who heads the party's House campaign committee, said he has warned colleagues to be prepared for an exceptionally challenging environment going into 2010.

But Van Hollen said voters will make their choices on the strength of the national economy and will reward Democrats for working aggressively to improve it.

"We passed an economic recovery bill with zero help from Republican colleagues," he said. "I think voters will see that and will ask themselves, 'Who was there to get the economy moving again, and who was standing in the way?'"

Democrats have gotten off to a much faster start than Republicans in fundraising for 2010. The Democratic Congressional Campaign Committee had $10.2 million in the bank at the end of July, with debts of $5.3 million. The National Republican Congressional Committee had just $4 million in cash and owed $2.75 million.

The economy poses the biggest problem for Democrats, with job losses of 2.4 million nationwide since Obama took office. Despite recent signs the country is pulling out of the recession, the unemployment rate in 15 states still was in double digits in July, led by Michigan at 15 percent.

Democrats must defend as many as 60 marginal seats next year, as opposed to about 40 for Republicans. Among those, about 27 Democratic and just 13 Republican seats are seen as especially ripe for a party switch.

Some involve incumbents stepping down to run for higher office.

For example, Rep. Joe Sestak, D-Penn., is mounting a primary challenge to Sen. Arlen Specter. Sestack's seat, until then safely Democratic, now becomes a top GOP target. The same goes for Louisiana Rep. Charlie Melancon, a Democrat in a GOP-leaning district who also is seeking a Senate seat.

But Republicans are on the losing side of that equation as well.