Nelson's drawn-out deliberations over whether to seek another term have spiked the blood pressure of national Democrats as they anxiously wait to see whether they stand a chance to hold on to the Senate seat in Nebraska. Nelson isn't a shoo-in if he does run again, but offers Democrats their best opportunity to hold the seat.

Although there is already a field of GOP candidates lining up to take on Nelson, Republican leaders, including Senate Minority Leader Mitch McConnellMitch McConnellTrump to meet with congressional leaders Monday: report Meet Trump's secret weapon on infrastructure Senate confirms first nominees of Trump era MORE (R-Ky.), are reportedly seeking to recruit Gov. Dave Heineman (R) to join the race, ostensibly to scare Nelson out of running.

Spending by Crossroads against Democratic candidates competing in 2012 races has become so prodigious that in many cases, it has dwarfed early spending by party committees and candidates' own campaigns. The group has pledged to spend $20 million on ads undercutting Obama.

Crossroads announced last week it was hitting Democratic Senate candidates in Missouri, Massachusetts, Montana and Nebraska with more than $1 million in ads. One month earlier, it spent $2 million in those states and $2.6 million in another five swing states to attack Democrats.

Watch the ad targeting Obama: