HUNTINGTON BEACH - The biggest surprise of this election might just come from Southern California, where many long-time incumbents are now facing their first serious challengers in years due to a combination of Democratic gains in voter registration, an unusually strong field of candidates and a nationally toxic environment for Republicans.

In particular, Democratic challengers in the 45th, 46th and 50th Congressional Districts have given U.S. Reps. Mary Bono Mack (R-Palm Springs), Dana Rohrabacher (R-Huntington Beach) and Brian Bilbray (R-Carlsbad) a run for their money.

One of the major reasons for these competitive contests has been the narrowing gap in registered voters between the parties. While Republicans still enjoy a substantial advantage over Democrats in all three districts, their leads have shrunk significantly.

Four years ago, Republicans led Democrats among registered voters by margins of 17 percent in the Orange County-based 46th, 15 percent in the San Diego-area 50th and 11 percent in the Riverside County-based 45th. By this year's registration deadline of Oct. 20, those leads had shrunk by 6 percent in the 50th, 5 percent in the 46th and 6 percent in the 45th.

Hoping to capitalize on these gains, Democrats recruited strong candidates that were well-suited for their respective districts.

In the 45th, Democratic candidate Julie Bornstein is running against Mary Bono Mack, a four-term incumbent. Bornstein is an experienced politician, having represented much of Riverside County in the state Assembly during the early 1990s. In addition, Bornstein served as the director of the California Department of Housing and Community Development under Gov. Gray Davis, and recently led The Campaign for Affordable Housing, a national nonprofit organization. Her background in the housing industry is particularly relevant in the 45th, home to the second highest foreclosure rate in the country (although it also served as the subject of a harsh attack ad by the Bono Mack campaign).

Bono Mack has won every one of her re-election campaigns by more than 20 points, but all signs point to a much closer race this time around. In fact, early voting numbers from the 45h Congressional District showed registered Republicans outnumbering Democrats by just five percent. In comparison, Republicans accounted for about 20 percent more of the early voters than Democrats in both 2004 and 2006.

In the 46th, Dana Rohrabacher is facing Democrat Debbie Cook, the most formidable opponent of his 18-year Congressional career. Cook is mayor of the district's most populous city, Huntington Beach, and has relied on a devoted network of grassroots supporters to counter Rohrabacher's substantial financial advantage.

Although Rohrabacher's always been a diligent campaigner, he has never faced a tough election. In fact, Cook's TV ads were the first by a Democratic challenger in the 46th in almost two decades.

Rohrabacher, though, is not taking his challenger lightly. In their only debate, Rohrabacher went after Cook repeatedly for allegedly flip-flopping on the Wall Street financial bailout package. In addition, the congressman has attacked Cook in numerous hard-hitting mailers.

Unlike the other two districts, the 50th is no stranger to a competitive campaign, at least in recent years. In the spring of 2006, Brian Bilbray squeezed out a victory over Democrat Francine Busby in a special election to replace disgraced former U.S. Rep. Randy "Duke" Cunningham.

Bilbray's current opponent, San Diego-area attorney Nick Leibham, boasts of a more organized and efficient campaign than Busby's five-month sprint in 2006. In addition, Leibham has proven to a be a relatively prolific fundraiser, outpacing Bilbray in both the 3rd and 4th fundraising quarters.

Leibham's substantial war chest allowed the Democratic challenger to air TV ads attacking Bilbray for much of the last two months. Bilbray, on the other hand, has stuck to positive ads on TV, saving the attacks for campaign mailers.

The wild card in all three congressional contests is the potential for depressed Republican turnout if the presidential contest turns into a landslide by early evening. As Rohrabacher recently told the Bay Area News Group, "If John McCain does not do well, and Republicans stay home, my lead could evaporate."

Some observers maintain that Republicans will turn out no matter what to vote on Proposition 8, which would ban gay marriages in California.

Even if many Republicans stay home, argues Gary Jacobson, a professor of political science at the University of  California, San Diego, no Democratic challenger in the last 40 years has overcome as huge a gap in voter registration as Leibham and Cook are facing. Jacobson, though, is the first to admit that "nothing's impossible."

In part because of the election's potential for defying historical precedent, political observers will be watching bellwether districts like California's 45th, 46th and 50th to see just how far the Democratic wave carries.

ELSEWHERE on PolitickerCA.com:

• Democrats close early voting gap in CA-45
• Bono Mack enters final stretch with big cash advantage over Bornstein
• Sparks fly at CA-46 debate
• Cook has best fundraising quarter, still trails Rohrabacher in cash on hand
• Republicans ahead in CA-50 early voting 
  
• Leibham pollster explains why Democrats can win CA-50