Sen. Al FrankenAl FrankenWhat killing net neutrality means for the internet Overnight Tech: Net neutrality fight descends into trench warfare | Zuckerberg visits Ford factory | Verizon shines light on cyber espionage Franken, top Dems blast FCC over net neutrality proposal MORE (D-Minn.) has kept up his torrid fundraising pace, pulling in $2.1 million in the final three months of 2013.

Franken now has $4.8 million in the bank for his reelection battle.

"We've got a tough fight ahead of us and these numbers demonstrate that Al is building an incredibly strong, people-powered grassroots campaign," said Franken campaign manager Matt Burgess. 

His top challenger, businessman Mike McFadden (R), has also been fundraising at a fast clip. McFadeen raised nearly $800,000 in the same three-month period and has $1.7 million in the bank for the campaign.

Franken's strong quarter belies a fast spending rate — he banked less than $1 million of the $2.1 million raised in the last quarter. Much of Franken's fundraising is done through direct mail, a process that can cost a lot but also pull in huge sums.

The freshman senator is considered the favorite for reelection, but Republicans hope a poor national climate for Democrats could give McFadden a chance at knocking him off next fall.