The head of the Appropriations committee said it would be "impossible."
An affiliate of Mark Zuckerberg's advocacy group is backing the N.H. senator.
Both candidates have 48 percent support, 50 days ahead of the election.
"[Y]ou have people like Jeanne Shaheen...making sure you stay open," he told workers.
The former Massachusetts senator's entrance gives the GOP another pickup opportunity.
Former senator makes surprise bid in race that may draw Scott Brown.
The Democratic incumbent held on to a must-win seat for her party on Tuesday night.
The former Massachusetts senator is using foreign policy to hit Sen. Jeanne Shaheen (D-N.H.).
The DSCC's first ad hits former Sen. Scott Brown (R-Mass.) for his Bay State ties.
The 2012 presidential nominee has endorsed and will campaign for the former Mass. senator.
Democrats say they now have better odds at winning seats in Montana and Louisiana.