In the next two weeks, Democratic leaders will review new polls and other data that show whether vulnerable incumbents have a path to victory. If not, the party is poised to redirect money to concentrate on trying to protect up to two dozen lawmakers who appear to be in the strongest position to fend off their challengers.
"We are going to have to win these races one by one," said Representative Chris Van Hollen of Maryland, chairman of the Democratic Congressional Campaign Committee, conceding that the party would ultimately cut loose members who had not gained ground.
With the midterm campaign entering its final two months, Democrats acknowledged that several races could quickly move out of their reach, including re-election bids by Representatives Betsy Markey of Colorado, Tom Perriello of Virginia, Mary Jo Kilroy of Ohio and Frank Kratovil Jr. of Maryland, whose districts were among the 55 Democrats won from Republicans in the last two election cycles.
Representatives John M. Spratt Jr. of South Carolina, chairman of the Budget Committee, and Earl Pomeroy of North Dakota, who is seeking a 10th term, are among the senior Democrats who have appeared to gain little ground in the summer months in the toxic political environment. A sputtering economy and discontent with Washington have created a high sense of voter unease that has also put control of the Senate in question.
To hold the line against Republicans, the House speaker,Nancy Pelosi, issued an urgent plea for members in safe districts to help their endangered colleagues by contributing money. She called out Democrats who were delinquent on paying their party dues and instructed members with no re-election worries to tap into a combined $218 million from their campaign accounts to help save their majority.
"We need to know your commitment," Ms. Pelosi wrote to lawmakers last week in a private letter, demanding that they call her within 72 hours to explain how they plan to help. She added, "The day after the election, we do not want to have any regrets."
A national campaign trumpeting Democratic accomplishments on health care, education and Wall Street regulation has given way to a race-by-race defensive strategy. Democratic incumbents are moving to aggressively define their Republican opponents and individualize races in an effort to inoculate themselves from the national mood.
In Missouri, Representative Ike Skelton has rarely run hard-hitting advertisements during 34 years in office, but he sternly accuses his opponent in one of not supporting the troops. In Texas, Representative Chet Edwards, using the word "lie" three times, accuses his rival in an advertisement of claiming that he voted in a recent election when polling records said he did not. In New Jersey, Representative John Adler accused his challenger, also in an advertisement, of buying a donkey so he could call his house a farm and get a tax break on it.
The sharp messages underscore the decreasing array of options facing Democrats as they try to overcome the party's declining standing. It is an open question whether voters will find the back-and-forth exchanges persuasive or whether some have reached the point where they are no longer listening to Democratic messages.
Nearly four dozen Democrats are broadcasting television advertisements — far more by Labor Day than in previous election cycles.
President Obama is adding more fund-raisers to his schedule, traveling to Florida and Minnesota over the next six weeks to try to build on the House Democrats' shrinking financial advantage over Republicans. Mr. Obama intends to increase his efforts to draw a contrast with Republicans as he does more campaigning in the coming weeks and tries to energize the party's core voters.