By Mark Greenbaum

Democrats' victory in a special congressional election this week was a very good result for Nancy Pelosi's caucus in a red district in western New York. But as analysts and the parties parse it for deeper meaning, they should be wary of reading too much into a special election.

Such races are generally decided by unique local factors that are unlikely to be mirrored in general elections dominated by national concerns. Narrow political cleavages, quirky election rules, and third-party candidacies can all play key roles in specials, and Tuesday's clearly follows this pattern.

While both parties have portrayed special elections as evidence of sizable gains to come, recent history is unclear on that.

In the last decade, there have been 41 special House elections that weren't held on the same day as regularly scheduled congressional elections. Nine resulted in a change of parties, with seven going from Republican to Democratic and two going the other way.

In a 2001 special election after Rep. Norman Sisisky (D., Va.) died, Republicans turned a Southern "yellow dog" district red. In 2004, Democrat Stephanie Herseth won South Dakota's sole congressional seat with a big assist from outgoing Republican Bill Janklow, whose vehicular manslaughter conviction cast a cloud over the race.

In 2008, Republicans lost reliable seats in Illinois and Louisiana thanks to weak GOP nominees. Later that year, Democrats impressively plucked Mississippi's conservative First District, but there, too, circumstances were unusual: A vicious Republican primary created rivalries between the district's east and west, giving Democrats an opening.

In 2009, a Conservative Party nominee in New York's historically Republican 23rd District seriously hurt the moderate GOP nominee, delivering it to Democrats. And in Hawaii's First District last spring, hokey election laws and a pair of stubborn Democratic candidates helped a Republican capture the blue seat with less than 40 percent of the vote.

All these races featured unique local circumstances that affected the outcomes more than any national issue. And so it was this week in New York's 26th, where, overcoming an early deficit, Democrat Kathy Hochul defeated Republican Jane Corwin. Hochul owes much of her victory to millionaire tea-party candidate Jack Davis, who undercut Corwin and helped Hochul win with a plurality. While the Medicare issue and other factors figured in the outcome, Davis was critical.

Consequently, those attributing greater meaning to Hochul's victory should do so carefully. In fact, if the fates of those who won other recent special elections is any guide, Hochul could have a short stay in Congress. The winners in Hawaii, Illinois, Louisiana, and Mississippi all promptly lost in 2010. New York's 26th is strongly Republican in lean and lineage, so it could follow suit.

As to whether Hochul's victory presages Democratic gains in 2012, history is even less clear. In 2008, after those Democratic wins in Illinois, Louisiana, and Mississippi, Democrats gained 21 seats in the House. And Republicans' May 1994 victories in Democratic districts in Kentucky and Oklahoma are now seen as having foreshadowed their House takeover that fall.

But other special elections haven't looked so prescient. In 2004, Democrats easily won GOP seats in special elections in Kentucky and South Dakota, only to do poorly that November. In late 2005 and early 2006, Republican victories in two California special elections preceded a 30-seat Democratic gain in November 2006. And in 2009 and early 2010, Democratic wins in highly competitive specials in New York and southwestern Pennsylvania came before Republicans' 63-seat shellacking in last year's midterms.

Judging by recent history, Democrats' victory in New York could be the first of many gains this cycle. But it could just as easily mean nothing.