By Bob Benenson, CQ Staff

Jigsaw Politics in July presented CQ Politics’ lists of the five seats most vulnerable to takeover by the challenging party, in three categories: the most vulnerable among all Senate and House seats, and the Democratic House seats most vulnerable to Republican challenges (which was included because the Senate and House Top 5’s, typical of this year’s partisan trend, were all Democratic bids to take over Republican seats).

Before we — and most of our readers — get swept up in the hoopla of the Democratic convention that starts soon, this and next week’s Jigsaw Politics columns are devoted to an update of those lists.

Today’s subject is the Senate list, which includes the biggest change from the original last month. The federal indictment of Republican Sen. Ted Stevens in late July, on charges of failing to report substantial gifts he allegedly received from a corporate ally, has greatly improved the Democrats’ odds of taking over the Alaska seat he has held for 40 years, boosting that race into the Top 5 list.

Alaska supplants Mississippi’s Senate special election in the Top 5. While still rated as a tossup by CQ Politics, the contest has been marked of late by some bad press for the Democratic nominee, former Gov. Ronnie Musgrove, who is challenging Republican Roger Wicker , a House veteran appointed last December to fill a vacant seat.

Republican campaign operatives have been trying throughout the campaign year to tie Musgrove to the principals in a scandal involving the failure of a beef plant that cost the state millions in payments it had raised for the design and building of the facility. Two executives of the Georgia company at the center of the scandal recently pleaded guilty of seeking to influence Musgrove in 2003 by making a $25,000 contribution — deemed by prosecutors as an “illegal gratuity” — to the treasury for his ultimately unsuccessful campaign for re-election as governor. This contribution, according to local news reports, came on top of a $20,000 donation the company executives had earlier made to Musgrove’s campaign funds.

Musgrove, who says Republicans are using the incident to smear him, has consistently denied any wrongdoing, contending that he had no direct role in choosing the company and that their campaign donations did not sway his administration’s decision-making on the beef plant. And Democrats have been trying to level the field by tying congressional campaign contributions Wicker has received to spending “earmarks” that he has sought as a lawmaker.

For comparison purposes and more details about the contests, the original Top 5 Vulnerable Senate Seats analysis can be found here.

Top 5 Vulnerable Senate Seats

Virginia: The presidential campaign of Illinois Sen. Barack Obama announced this week that former Virginia Gov. Mark Warner — who is running to succeed retiring five-term Republican Sen. John W. Warner — will be the keynote speaker at the Democratic National Convention being held the last week of this month. The move was seen as a signal that Obama is following through on plans to seriously compete for the 13 electoral votes in Virginia, which has gone Republican in each of the past 10 presidential elections.

But the Democrats would not be doing this if they also weren’t fully confident that Mark Warner is going to win his Senate race, just as they felt certain about Obama’s prospects in his 2004 Senate bid when they picked him for a keynote speech that launched his meteoric rise in national politics.

And it again looks like the Democrats are making a pretty safe bet. Warner — whose highly popular tenure as governor from 2002 to 2006 set the stage for other significant Democratic victories in a state that long had been trending Republican — is heavily favored over his gubernatorial predecessor, Republican James S. Gilmore III, with the most recent poll earlier this month showing the Democrat with a whopping 24 percentage-point lead.

New Mexico: Nothing has changed of yet to shake the perception that five-term Democratic Rep. Tom Udall has a solid edge over Republican Rep. Steve Pearce in the contest to succeed retiring six-term Republican Sen. Pete V. Domenici . While Udall was unopposed for the Democratic Senate nomination, Pearce made the finals by narrowly winning a bruising primary, and the congressman faces a tough obstacle in winning over those supporters of Rep. Heather A. Wilson , his more centrist GOP opponent, who argued during the primary campaign that Pearce is too conservative to win a highly competitive swing state such as New Mexico.

Alaska: FBI and IRS agents raided Stevens’ Alaska home in July 2007 as part of a sweeping political corruption investigation involving the Alaska-based Veco oil field services company. This, in turn, emboldened Anchorage Mayor Mark Begich, the most prominent Democratic officeholder in the strongly Republican-leaning state, to launch a challenge to Stevens after a series of elections in which his party essentially ceded the seat.