Longtime Democratic incumbents are a seriously endangered species.

In House races, they are trailing their Republican challengers by margins that suggest their careers are about to be extinguished, according to The Hill 2010 Midterm Election Poll.

Budget Committee Chairman John Spratt (D-S.C.) is down 10 percentage points, while Reps. Allen Boyd (D-Fla.) and Chet Edwards (D-Texas) are each losing by 12. Rep. Paul Kanjorski (D-Pa.), who is serving his 13th term, is trailing by five.

Republicans have targeted all of these Democrats before, but are closer than ever to ousting them.

Edwards has served two decades in Congress, while Boyd — who barely survived a tough primary challenge earlier this year — has been in the House since 1997. Spratt is the most senior of the group, having first been elected in 1982.

In most cycles, lawmakers who have served in Congress for more than 10 years are considered safe bets to win reelection. But this is far from a usual election cycle. Experience is a handicap where once it was an advantage.

In each of these districts, likely voters were asked, “Your current member of Congress has been in Washington for several years. Do you consider his years to be: A reason to vote for him; or a reason to vote against him?”

A majority of the polled voters in each of the four longest-serving incumbents’ districts said it was a reason to vote against the lawmaker, most strikingly in Boyd’s district, where the breakdown was 33 for, 52 against.

Kanjorski and Edwards are subcommittee chairmen; Boyd has been a leader of the Blue Dog Coalition in the House.

Spratt, Boyd and Edwards all hail from districts won by Sen. John McCain (R-Ariz.) in the 2008 presidential election contest, while Obama is far less popular in Kanjorski’s district than he was two years ago, according to The Hill’s poll. Obama captured 57 percent of the 2008 vote in Kanjorski’s district, but now only 47 percent approve of the job the president is doing, while 51 percent disapprove.

Spratt, Boyd and Kanjorski all voted for healthcare reform and climate change legislation; Edwards rejected both bills. McCain won 67 percent of the vote in Edwards’s district, which encompasses former President George W. Bush’s ranch home in Crawford.

Other longtime Democratic incumbents Republicans are targeting this year include Armed Services Committee Chairman Ike Skelton (Mo.) and Reps. John Dingell (Mich.) and Rick Boucher (Va.).

Meanwhile, Rep. John Salazar (D-Colo.), who is in his third term, is trailing Scott Tipton by four points, though the race is within the margin of error of 4.9 percent, according to The Hill’s new polling data. Four-term Rep. Jim Marshall (D-Ga.), meanwhile, is losing his race to Republican Austin Scott, 50 percent to 37.

All six of the 10 incumbents who are trailing in the new poll attracted 43 percent of the vote or less in their respective districts. Campaign experts say that incumbents who are below 50 percent just before Election Day are probably headed for defeat.

While most Democrats are freefalling in polls, Rep. Earl Pomeroy (D-N.D.) is on the rise. Down in the polls earlier this year, Pomeroy — who opposed climate change and backed healthcare reform — is leading in his race by one point against GOP challenger Rick Berg.

Pomeroy’s experience in Washington is actually helping him. Forty-five percent of surveyed voters said his time in Washington is a reason to vote for him, while 35 percent said it’s a reason to vote against the 58-year-old lawmaker.

Pomeroy, who serves on the influential Ways and Means Committee, is serving his ninth term.

The National Republican Congressional Committee (NRCC) is spending a lot of money to defeat long-term incumbents. It has spent more than $500,000 in independent expenditures in each race against Boyd and Edwards. The NRCC has spent more than $1 million in Spratt’s and Pomeroy’s districts and more than $900,000 against Kanjorski.

The Democratic Congressional Campaign Committee has spent less than the NRCC in each of these districts. It has invested the most to save Spratt, Pomeroy and Kanjorski, while spending far less in Boyd’s and Edwards’s districts.

By Bob Cusack, “The Hill”