She likened the situation to learning that Sen. Kent Conrad, the last remaining Democrat in North Dakota's congressional delegation, would retire in 2012.

Initially, it looked like Democrats would have next to no chance to be competitive in the race for Conrad's replacement. But the entrance of former state Attorney General Heidi HeitkampHeidi HeitkampUnder pressure, Dems hold back Gorsuch support Overnight Finance: Senators spar over Wall Street at SEC pick's hearing | New CBO score for ObamaCare bill | Agency signs off on Trump DC hotel lease The Hill’s Whip List: Where Dems stand on Trump’s Supreme Court nominee MORE (D) has turned the race into a toss-up, and Democrats are now bullish about holding on to that seat.

Murray also defended the DSCC's decision to spend early and heavily on ads bolstering Nelson, despite his reluctance to make his caucus aware of his reelection plans and the possibility that the spending would be in vain if he decides to call it quits.

"Ben Nelson was being attacked viciously," she said, referring to attack ads aired by outside conservative group Crossroads GPS targeting Nelson. "We're not going to sit back and watch one of our senators just be inundated and not support them."