By Bob Benenson, CQ Staff

This is clearly another very challenging year for the national Republican Party in congressional politics, on the heels of the big losses in 2006 that cost the GOP its Senate and House majorities. The past two Jigsaw Politics columns presented CQ Politics’ lists of the five Senate seats and the five House races most vulnerable to partisan takeover this year — and all are Democratic bids to capture Republican seats.

It would, nonetheless, be impossible for the House GOP to do worse in the takeover game than in 2006, if only because you can’t get lower than zero. The Democrats gained 30 seats that had been held by Republicans while defending every single one of their own that they brought into the elections. (What made the Democrats’ upsurge even more remarkable was that the party also ran the table in the races for the Senate and governor’s offices, picking up six of each while losing none of their own.)

A clean sweep such as the one two years ago is exceeding rare — and therefore unlikely to be repeated this year, even though the overall political atmosphere appears again to give a big advantage to the Democrats. Even in 1994, when the Republicans scored the biggest House gain in the past 60 years by netting 52 seats, the Democrats took over four seats that had been left open by Republicans who did not run for re-election.

While CQ Politics currently rates 53 Republican House seats as subject this year to Democratic takeover bids ranging from serious to longshot, there are 36 Democratic seats similarly in play.

The following is Jigsaw Politics’ take on the five of those Democratic seats that appear most vulnerable of falling to the GOP this fall, with links to more detailed information on each race:

• Louisiana’s 6th District, where Democratic Rep. Don Cazayoux faces a difficult short-turnaround defense of the formerly Republican seat he just won in a May 3 special election. It was a stunning setback for Republicans who thought they would hold on to the seat being vacated by 10-term GOP Rep. Richard H. Baker ’s February resignation. But the GOP got what appears one of their biggest breaks in any race this year just before the July 11 filing deadline: Democratic state Rep. Michael Jackson, who has appeal among the large constituency of his fellow African-Americans in the Baton Rouge-based district, filed as an independent after losing to Cazayoux in the April 5 special election primary runoff. Republicans also express confidence that their candidate, state Sen. Bill Cassidy, will run better than their special election nominee, Woody Jenkins, a former state representative with a checkered history as a candidate, who lost to Cazayoux by 49 percent to 46 percent.

• Kansas’ 2nd District, a longtime Republican stronghold where freshman Democrat Nancy Boyda is defending a seat she won in a 2006 upset over five-term Republican incumbent Jim Ryun. Boyda, a former pharmaceutical company executive (and former Republican), is well-funded and continues to strike the profile as a Democratic moderate that she ran on in her win two years ago. But there surely is fodder in her House voting record for the GOP to make a case that she is not the kind of conservative representative who folks want in an eastern Kansas district that gave 59 percent of its votes to President Bush in 2004. The Republicans’ problem here — ironic, in the sense that the party has had huge candidate recruiting problems in many key races — is that they have not one, but two strong contenders seeking to take on Boyda. And the campaign for the Aug. 5 primary between the comeback-seeking Ryun and state Treasurer Lynn Jenkins has been expensive and bruising.

• Florida’s 16th District, where freshman Democrat Tim Mahoney was boosted to victory in 2006 by what was perhaps the year’s most bizarre political development. Although Mahoney, the wealthy owner of an investment banking firm, had invested heavily of his own money in his challenge that year, he appeared an overwhelming underdog in the Republican-leaning south-central Florida district — until Republican Rep. Mark Foley, then the popular six-term incumbent, resigned his seat and quit the race in late September after revelations of sexually explicit messages he had sent to teenage pages. Although Republican officials substituted a state lawmaker as their nominee, Foley’s name remained on the ballot, and the GOP has claimed since that Mahoney’s 2 percentage-point win was solely the result of the scandal. Mahoney, who positions himself as a center-right Democrat, argues otherwise, and again will be very well-financed. And here again, the GOP faces a late primary problem, with three serious contenders battling for the Aug. 26 nominating contest.

• Alabama’s 5th District, where the Republicans have a rare open-seat takeover opportunity in a year when the vast majority of voluntarily departing House members are Republicans. The 5th is conservative northern Alabama turf that regularly gives Republicans wide margins for president and key statewide offices, so GOP officials saw a golden opportunity when nine-term Rep. Robert E. “Bud” Cramer — a popular conservative-leaning Democrat — made his surprise announcement that he would retire. But as with so many circumstances facing the Republicans, the path in Alabama 5 hasn’t been smooth. Cramer’s March announcement came just three weeks before the candidate filing deadline. While his party settled quickly on Parker Griffith, a state senator with an ideology profile similar to Cramer’s, the Republicans had a recruiting scramble in which some sought-after potential candidates opted out. Their nominee, insurance executive Wayne Parker, appears a solid contender, but he fell short of the majority needed to win outright in the June 3 primary and had to expend additional time and resources before easily winning the July 15 runoff. Parker nearly upset Cramer in a House election, but that was back in 1994, and he took a long hiatus from House politics after losing by a wide margin in a 1996 rematch.

• Texas’ 22nd District, a seat that turned over to the Democrats in 2006 in one of the most frustrating and symbolic losses for the Republicans. Although this mainly suburban and strongly conservative Houston-area district gave 64 percent to Bush in 2004, Democrat Nick Lampson made a congressional comeback in 2006 by winning the seat long held by Republican powerhouse Tom DeLay, the former majority leader, who resigned that July in the midst of serial ethics controversies. It was sweet revenge for Lampson, whose previous four-term House tenure ended in the 2004 election after a mid-decade Republican redistricting push, spearheaded by DeLay, shifted his political base away from him. Lampson, reflecting the stronger Republican lean of his new district, has differed more often than previously with most of his fellow House Democrats, and he has raised lots of money for his re-election bid. But Republicans continue to portray his 2006 win as a fluke, and they have a solid nominee in Peter G. Olson, a former chief of staff to Texas Republican Sen. John Cornyn .

All of these races currently are rated No Clear Favorite by CQ Politics except Texas 22, which has a rating — admittedly marginal — of Leans Democrat.

The Texas race received the fifth slot over a contest, in Oregon’s 5th District, that has a rating of No Clear Favorite that is under reconsideration by CQ Politics. Six-term Democratic Rep. Darlene Hooley is retiring, setting up what should be a welcome takeover opportunity for the GOP in a district that Bush won narrowly in 2004. Their nominee, businessman Mike Erickson, made a decent showing as a political newcomer in losing to Hooley by 11 points in 2006, and has exhibited a virtually limitless willingness to spend his own money on his campaigns. But an opponent in the May 20 Republican primary broadcast an allegation that Erickson, an anti-abortion candidate, once drove a girlfriend to an abortion clinic and gave her money, and the controversy exploded onto the front pages of Portland’s Oregonian newspaper when the woman in question went public with that accusation.

Erickson contends that he did not intend to pay for an abortion and is the victim of a smear campaign. But some prominent Oregon Republicans have distanced themselves from the candidate since the furor erupted. The Democrats have a politically experienced candidate in state Sen. Kurt Schrader.