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 $757 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.
"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 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. Two Republicans in heavily Democratic districts - Reps. Mark Kirk of Illinois and Joe Gerlach of Pennsylvania - are vacating their seats to run for Senate and governor, respectively.
At least one Republican is considered extremely vulnerable: Joseph Cao of Louisiana, who defeated Democrat William Jefferson after the nine-term incumbent was indicted on corruption charges. The district, which includes most of New Orleans, is considered one of the most Democratic in the country.
Beyond that, most of the closest races involve Democrats who rode the Obama tide in 2008.
They include at least four in Ohio, a perennial presidential swing state that has been battered for years by a persistently weak economy. Two represent bellwether areas: Rep. Mary Jo Kilroy, whose district covers most of Columbus and its suburbs, and Rep. Steve Driehaus, whose district includes much of Cincinnati and its suburbs.
Each won with the help of a strong showing among Obama supporters, and each faces face a rematch with the candidate who narrowly lost last year.
"I don't know if Kilroy or Driehaus have any particular problems, but we have a bad economy, the president's popularity has gone down, and conservatives are aroused and angry about government spending, cap and trade and the health care plan," said John Green, a political science professor at the University of Akron.
Indeed, the "cap and trade" bill that narrowly passed the House last spring is creating headaches for several Democrats. The legislation, which would cap carbon emissions and tax industries that exceed the cap as a way to reduce global warming, is largely unpopular in areas of the country where jobs rely on oil, gas or coal production.
One Democrat most affected is New Mexico Democrat Harry Teague. His district, which McCain carried last year, is one of the largest oil and gas producing areas in the country, and Teague has faced angry crowds back home ever since voting yes.
Teague will face Republican Steve Pearce, who held the seat for three terms before giving it up to run unsuccessfully for the Senate last year.
Without Obama on the ticket, a lower predicted black turnout in 2010 could also affect Democrats in several tight races in the South. These include Reps. Bobby Bright and Parker Griffith of Alabama, Travis Childers of Mississippi, and Tom Perriello of Virginia, who won by just 745 votes last year in a district that is 24 percent black.
Concerns about Obama's health care plan and the mounting federal debt could ensnare two first-term Florida Democrats, Alan Grayson and Suzanne Kosmas. Both represent districts along the state's competitive I-4 corridor, which is heavily populated by independent voters and retirees. Polls show Obama has lost ground among both of those demographic groups nationwide.
Copyright 2009 The Associated Press.