In Tuesday night's special election to replace Anthony Weiner, the New York congressman brought down by his extramarital Facebook chats, Democrats lost a seat they've held for 90 years. Naturally, they insist the loss means nothing--the results "are not reflective of what will happen in November 2012," Representative Steve Israel of Long Island said, according to The New York Times' Michael D. Shear. And Republicans, of course, insist their win means everything. "This clear rebuke of President Obama’s policies delivers a blow to Democrats' goal of making Nancy Pelosi the speaker again," Representative Pete Sessions of Texas said. While the results obviously don't predict what will happen in 2012, most people say they're a bad omen for Democrats. But why it's bad omen is still in dispute.

Why were Democrats crushed? A breakdown of the competing theories:

Bad candidate

Democrats prefer this one. Democrat David Weprin is a state legislator from an important Democratic family; his opponent, Bob Turner, is a charismatic outsider--a former producer for the Jerry Springer show. Weprin made a few gaffes--for example, even though Washington was obsessed with the debt ceiling all summer, he still underestimated the size of the national debt by $10 trillion. Politico's Dan Hirschhorn and Alex Isenstadt note that Weprin also skipped a a debate, using Hurricane Irene as an excuse even though the storm had already passed. But Turner made some mistakes too, like saying he'd never met a tax loophole he didn't like.

Jewish voters don't like Obama's policies toward Israel
 
This one is clearly Matt Drudge's favorite:
 

The Brooklyn and Queens district has a lot of Orthodox Jewish voters, and a majority of them disapprove of Obama's job performance, a recent poll found. And Turner portrayed Weprin as agreeing with Obama that Israel should return to its pre-1967 borders. Former New York City Mayor Ed Koch urged voters to "send [Obama] a message" on his Israel policy--even though Weprin, an Orthodox Jew himself, opposed the idea of using the 1967 borders as a starting point for negotiations with Palestinians.

Voters are mad about the economy
 

The plurality of voters--32 percent--said the economic recovery was the most important factor in their vote. Voters blame a bad economy on the guy in office, and Turner exploited that, The Atlantic's Nancy Scola explains, handing out flyers with a photo of Obama chilling in the Oval Office over the tag line, "Obama thinks he can fix the economy on a bus. He already threw Israel under it."

Conservative Jews don't like gay marriage either

Politico's Ben Smith reported the day of the election:
People who work in New York's Orthodox Jewish politics say that David Weprin's support for same-sex marriage has been a major issue in the district, whose Orthodox Jews aren't the closely-knit Chasids who don't care much about which outsiders marry one another, but more traditional social conservative voters.
According to Public Policy Polling, 29 percent of likely voters in the district thought gay marriage was "very important" in deciding who to vote for. Smith says the issue likely made a difference on the margins.
 
When Democrats back entitlement cuts, it's hard to use them as a wedge
 

Weprin tried to use the playbook used by Democrats in this spring's special election in upstate New York, in which Kathy Hochul won in a traditionally Republican district by tying her opponent to House Republicans' budget plan that called for turning Medicare into a voucher program. But the issue didn't really stick. Slate's Dave Weigel explains that Turner's campaign was able to fight back against attacks that he'd cut Medicare and Social Security -- and Democratic state assemblyman Dov Hikind didn't help make his party's case when he said the White House was trying to cut those programs, too. Weigel continues:

That's not really a wedge issue -- it's the slow death of a wedge issue. It's the start of a problem for Democrats, who have gone from attacking the Ryan plans for entitlement reform to vouching support for some undefined "everything on the table" entitlement reform. There might not be any way for Democrats to dodge this, and there's no sign that they want to.

Whatever the explanation, The New York Times' Nate Silver reports that last night's numbers--combined with a more-expected Republican victory in Nevada--are a bad omen for Democrats. 

One crude way to forecast the results you might expect to see out of a House race is through its Partisan Voting Index, or P.V.I., a measure of how the district voted relative to others in the past two presidential elections.
 
The Nevada Second, for instance, has a P.V.I. of Republican plus-5, meaning that the Republican candidate would be expected to perform 5 points better there than a Republican might nationally. Since a vote for the Republican is (usually) a vote against the Democrat, you need to double that number to project the margin of victory. In this case, that would imply a Republican win by 10 points given average candidates and a neutral overall political environment.
 
The Republican Mark Amodei, however, leads by 22 points as of this writing, an easy victory, meaning that he overperformed the P.V.I. by 12 points.
 
Meanwhile, Mr. Turner's winning margin in the New York district, 8 percentage points as of this writing, represents a 18-point G.O.P. swing from the P.V.I.-projected results.
In Hochul's upstate New York victory, her win was a 17-point swing for Democrats. But even if you add that, and a Democratic win in California in July, to recent special election results, Republicans are still overperforming by an average of 7 points, Silver says. Last fall, Republicans won back a majority in the House of Representatives by an average of a 7 point lead in the popular vote.