Vulnerable Democrats beware: The bipartisan well is about dry. 

Last cycle, Democrats saw their percentage of the Republican vote drop dramatically, and that could spell trouble for incumbents relying on that vote to survive in 2012. Sen. Ben Nelson is at the top of the list.

The Nebraska Democrat received a whopping 42 percent of the GOP vote in 2006 when he cruised to re-election, but with two GOP statewide officeholders already running against him, the Senator's narrow victory in 2000 is a better road map for a third term. The former two-term governor was elected to the Senate by just a couple of points with the help of 21 percent of the Republican vote.

The trouble for Nelson is that no Democratic Senator or nominee in a competitive race even came close to that share of the GOP vote last cycle, according to an exit poll analysis. As the country becomes more polarized, partisan voters are going against politicians they may like or have supported in the past because they are upset with the candidate's national party. 

In Arkansas, Sen. Blanche Lincoln (D) saw her percentage of the Republican vote slip from 12 percent in 2004 to just 4 percent last cycle when she lost re-election.

In Wisconsin, Sen. Russ Feingold (D) dropped from 14 percent of the GOP vote in 2004 to 5 percent last fall when he lost re-election.

But there may be some hope for vulnerable Democrats.

"It matters less what you get with Republicans," said Democratic pollster Fred Yang of the Garin-Hart-Yang Research Group. "It's more important how well you do with independents." For example, while Feingold's GOP vote fell, his share of independents plummeted from 62 percent in 2004 to just 43 percent last fall.

For the most vulnerable Senators, putting together a winning coalition is complicated and tenuous. 

In Missouri, Secretary of State Robin Carnahan (D) received just 4 percent of the GOP vote in her loss last year to now-Sen. Roy Blunt (R). This cycle, Sen. Claire McCaskill (D) would need only 5 percent of the Republican vote (she won 7 percent in her 3-point victory in 2006), assuming that Democrats and Republicans each make up 37 percent of the electorate - a reasonable expectation based on recent elections - and that the Senator earns 54 percent of the vote from independents.

That doesn't seem insurmountable, but that means McCaskill would have to improve from her 2006 showing with independents (51 percent, in a good Democratic year), and considering Carnahan pulled just 31 percent of independents in November.

It's debatable whether McCaskill has carved out her own centrist image compared with Carnahan. But unlike Carnahan, McCaskill is a sitting Senator who has had to take tough votes, including several in favor of President Barack Obama's health care law.

For many Democratic incumbents, there just isn't a lot of room for error.

In 2006, Jon Tester (D) defeated Sen. Conrad Burns (R) by just one point with the help of 11 percent of the GOP vote in Montana. Using 2010 as a guide, Tester could get that Republican chunk again if he were running against a polarizing opponent.

Last fall, Senate Majority Leader Harry Reid (D) drew 11 percent of the GOP vote in Nevada, now-Sen. Chris Coons (D) got 15 percent in Delaware, now-Sen. Richard Blumenthal (D) garnered 16 percent in Connecticut, and now-Sen. Joe Manchin (D) got 17 percent of Republicans in West Virginia. They all defeated Republican Senate nominees with considerable liabilities.

But Tester faces Rep. Denny Rehberg (R), who already has won statewide as the state's at-large Representative.

Sen. Bill Nelson (D) benefited from facing former Florida Secretary of State Katherine Harris (R) in 2006. He won 24 percent of the Republican vote against the then-Congresswoman, up from 13 percent when he first won in 2000. Several Republicans will square off in a competitive primary in hopes of facing Nelson in 2012.

It's unclear whether this trend could widen the Senate playing field for Republicans. Sen. Maria Cantwell (D-Wash.) received 11 percent of the GOP vote when she was elected by less than a point in 2000. She received 9 percent in her easier 2006 re-election. Last year, Sen. Patty Murray (D-Wash.) won re-election, but with just 4 percent of the Republican vote.

In Wisconsin, Sen. Herb Kohl (D) has enjoyed support from more than a quarter of Republicans in his past two elections, but not one Democratic candidate in a Senate race with an exit poll achieved that last year.

Fortunately for Senators, they have the opportunity to make the race a choice between two candidates, whereas House races are much more partisan.

Over the past four election cycles, GOP candidates have captured a remarkably similar chunk of the Democratic vote. After garnering 9 percent crossover support in 2004, GOP candidates have taken 7 percent of the Democratic vote in each of the past three elections, according to the national House exit poll. In contrast, Democratic House candidates went from 7 percent of the GOP vote in 2004 to 8 percent in 2006, up again to 9 percent in 2008 and plummeting to 5 percent in 2010. 

Of course, Republicans aren't immune to losing Democratic votes.

Sen. Scott Brown (R) presumably won his January 2010 special election in Massachusetts with the help of some Democrats and left-leaning independents. It will be tougher for Brown to capture those voters in a presidential year, when he is up for re-election to a full six-year term. (There were no exit polls in his initial race to determine how steep of a climb he has.)

The frustrating part for these incumbents is their lack of control in their own races. Incumbents can try to demonize their opponents, but they can still be subject to the national mood defining their own party.