Democrats hold a 53-47 advantage in the Senate. They are defending 23 seats to Republicans' ten next year. Looking at the state-by-state map, the math is even tougher. Eight of the Cook Political Report's ten races designated as "tossups" (including Nebraska) are controlled by Democrats right now. Democrats spent over a million dollars during the off year running ads aimed at shoring up Nelson's standing in the state. Republicans returned fire late in 2011 with ad buys from several outside groups including six figures from Karl Rove's Crossroads GPS. Barring a surprise bid from Bob Kerrey, the former Democratic senator and governor who has not ruled out a run, Democrats have virtually no bench to speak of in the Cornhusker State, and thus the race becomes an almost certain Republican pickup. This at least means Democrats won't likely be pouring millions of dollars into a race they would have had, at best, a 50-50 shot of winning. But Republicans too, will now not have to go on offense as heavily in Nebraska. The Republican field in Nebraska has been anything but impressive, with frontrunning Attorney General Jon Bruning suffering through his share of missteps and mediocre fundraising. What's more, national conservative figures have fanned the flames, with Sens. Jim DeMint, R-S.C., and Mike Lee, R-Utah, throwing their support to underdog candidate Don Stenberg, the state Treasurer who lost to Nelson in 2000. But without a formidable Democratic successor waiting in the wings, this doesn't matter nearly as much as it did. Democratic Senatorial Campaign Committee Chair Patty Murray chose her words carefully on Monday, opting to express confidence Democrats will remain competitive in Nebraska, but stopping short of predicting a victory. "Republicans will continue to have their hands full with a very divisive primary in the state, which will provide an opportunity for Democrats to remain competitive," she said. Much of the anger directed at Nelson has been over his support for President Obama's most hard-fought -- and controversial -- measures, including the stimulus package and the Democratic-backed health care law. Now, control of the Senate will rest in the hands of other key swing state Democratic incumbents who will also have to explain their support for key elements of Obama's agenda. Sens. Claire McCaskill, D-Mo., and Jon Tester, D-Mont., both face tough reelection fights in states where the president isn't likely to fare well next year. If they can overcome what hamstrung Nelson so badly in Nebraska, Democrats as a whole are likely to prevail next November. If they fall victim to the same kinds of attacks that persuaded Nelson not to run next year, it could be a long Election Night for the party. Beyond Montana and Missouri, open seat races in Wisconsin and Virginia will also be a test of how well Democrats who have stuck close to the president can perform in swing states. And Republicans are bullish about pickup opportunities in North Dakota and Hawaii. There are encouraging signs for Democrats. In the two races where the party stands a serious chance of picking up a seat -- Nevada and Massachusetts - recruiting has yielded candidates that have run solid campaigns so far. And the lack of top notch Republican gets in places like Florida, Missouri, and Pennsylvania have been an unexpected blessing for the party. Privately, some Republican strategists have expressed newfound doubt over the past few months that their party will be able to win back control of the upper chamber, despite early optimism. But Nelson's departure is another domino that has fallen in the GOP's favor. "Simply put: It is time to move on," Nelson declared in a statement explaining his decision on Monday. For Democrats, too, it is time to move on, from a race in which they invested significant resources and time. But they will have to fight the same battle in several other states, face the same negative ads and the same lines of attack. They're hoping the outcome is different.