Already unable to mount a proper defense of the 40-plus GOP House seats that are obviously under threat of a Democratic takeover, Republican incumbents are now scrambling to avoid upsets in at least five districts that were previously deemed safe.

In Arizona, California, Indiana, Nebraska and Texas, Republicans are nervous about losing House districts long considered solidly conservative. The incumbents imperiled are Reps. John Shadegg (Ariz.), Mark Souder (Ind.) and Lee Terry (Neb.); Reps. Michael McCaul (Texas) and Dana Rohrabacher (Calif.) are classified as marginally in jeopardy.

Making these seats vulnerable are a political environment that has gone from difficult to toxic for the GOP, campaign operations and incumbent candidates that in some instances are unprepared to run a competitive race, and in some cases, an infusion of national Democratic money. The National Republican Congressional Committee, acknowledging reality, declined to dismiss talk of GOP upsets as simply a Democratic talking point.

“This election, more than most, is very volatile. Republicans have a much larger battlefield to protect. We’re being outspent 4-1, and the political environment is like none we’ve ever seen,” NRCC spokeswoman Karen Hanretty said. “We’ve encouraged Republican incumbents all along to take nothing for granted.”

Of these latest potential upsets, Democrats appear to be most enthused about attorney Bob Lord (D), who is challenging Shadegg in Arizona’s suburban Phoenix 3rd district, and businessman and 2006 nominee Jim Esch (D), who is in a rematch with Terry in Nebraska’s Omaha-area 2nd district. The DCCC has already reported independent expenditures of nearly $680,000 on Lord’s behalf, and the committee released a poll there late Friday that showed the race tied.

Also late last week, the DCCC reserved a nearly $450,000 media buy for Esch, with television ads set to run for the duration of the campaign.

Next on the committee’s radar is Souder, who appears to be locked in a tight race with attorney Michael Montagano (D) in Indiana’s 3rd district. Democratic sources say the DCCC could be interested in investing in Montagano — and possibly in Missouri’s solidly conservative, open 9th district, where state Rep. Judy Baker (D) is running against former state Rep. Blaine Luetkemeyer (R).

These same sources say the DCCC is less inclined to get involved in Texas’ 10th district, where McCaul is facing attorney and television personality Larry Joe Doherty (D), and in California’s 46th district, where Rohrabacher (Calif.) is being challenged by Huntington Beach Mayor Debbie Cook (D). Sources say McCaul and Rohrabacher have taken steps to bolster their political standing, while their challengers have failed to gain traction.

Among this group of potential upsets, Souder might be the most vulnerable. His campaign operation appears to have engendered the most criticism among Republican operatives familiar with these newly competitive races.

Montagano has raised more money than Souder, has made inroads with the 3rd district’s moderate and business-minded Republicans who have never warmed to Souder’s strident conservatism and has developed a reputation for being personable.

Souder has never faced a competitive general election race, has never been a big fundraiser and is viewed as abrasive. One Indiana-based GOP operative expects the strong conservative lean of the Fort Wayne-area district and the presidential turnout to ultimately save Souder’s job.

But he said the Congressman isn’t doing himself any favors with his advertising, and he added that media there would be cheap for the DCCC to purchase. Democratic presidential nominee Sen. Barack Obama (Ill.) is making a strong play for Indiana’s electoral votes, and his organization might also aid Montagano on Election Day.

“I think Souder’s under 50, and I think it’s in single digits,” the Indiana Republican operative said. “Do I think it’s in the margin [of error] yet? No. But Montagano doesn’t have to be in the margin yet. There are still three weeks left.”

The Souder campaign was unavailable for comment at press time Friday.

The Montagano campaign is optimistic and touted a poll that showed the Democratic attorney trailing Souder by just 5 points, 44 percent to 39 percent. The Montagano poll of 503 likely voters was taken Oct. 6-7 and had an error margin of 4.4 points.

“It’s an extremely competitive race,” Montagano campaign spokesman Daynan Crull said. “Mark Souder’s never been this close to losing this seat since he’s been in Congress.”

In Nebraska’s 2nd district, Terry might be in trouble, but he is not going to be surprised.

The Republican incumbent has been waging an aggressive campaign for months, both on the ground and in the media. Terry has raised enough money for his team to believe that he can withstand attacks from the DCCC, the Obama campaign and any third-party group that might decide to jump in on Esch’s behalf — even absent help from the NRCC.

Some Republican operatives familiar with Terry’s race have questioned Terry’s campaign tactics, saying his advertising lacks polish and his ground game has not always focused on the highest-propensity GOP voters. Obama descending on the 2nd district in a quest for one electoral vote has also raised the stakes for Terry — in Nebraska, the presidential candidate who wins each Congressional district wins that district’s electoral vote.

But according to one Nebraska Republican, Terry is likely to win by a more comfortable margin than the current state of the race might suggest.

Former Nebraska GOP Chairman David Kramer conceded that Terry is in the midst of a competitive contest. But Kramer said Esch isn’t ready for prime time nor is he the kind of conservative Democrat who can appeal to the conservative-leaning 2nd district. Kramer also argued that Terry’s campaign has been more effective than some observers give it credit for.

Kramer suggested that Terry’s campaign operation might be receiving criticism because the Congressman’s team last cycle was perceived to have done a less-than-stellar job against Esch, who held Terry to 55 percent — his lowest winning percentage ever.

“I don’t think [Terry] is in as much trouble as a lot of folks want to make it sound,” Kramer said. “I don’t sense enough disaffection with Lee across the political spectrum that they would vote him out.”

The Terry campaign agreed with Kramer’s analysis. Dave Boomer, Terry’s campaign manager, said he has confidence in his operation and expects it to pay dividends on Election Day.

Republicans have readily conceded that their Congressional candidates faced a difficult Nov. 4. President Bush’s low approval ratings and the DCCC’s $40 million cash lead over the NRCC as of Aug. 31 were factors that loomed large in the upcoming elections.

But Republican strategists who focus on Congressional races say the two-week debate over a federal rescue of the financial markets and an economy that continues to worsen have hit at the worst possible time. Compounding the problem for GOP incumbents and helping fuel the fire for potential upsets in House races is Bush’s very public presence since the economic meltdown began.

“He’s on TV every other day, reminding voters how little they trust him, and how much they blame the Republican Party for everything that’s wrong,” one GOP operative said.