He goes on to warn that in 2008, before the Citizens United Supreme Court case provided for the creation of super PACs, "millions of dollars in outside funding poured into Colorado to stop my bid for U.S. Senate." His race saw a high influx of outside money in 2008, but he managed to win in purple Colorado with a 10-percentage-point lead over his Republican challenger.

It's unclear where Udall is calculating the sum, but he says that his campaign is "anticipating upwards of $20 million from super-PACs run by Karl Rove, the Koch brothers, and other allies of the obstructionist right wing of the Republican party."

"Can you contribute $25 before MIDNIGHT TOMORROW towards our $10,000 year-end goal so we can start building the grassroots network we’ll need to fight back?" he goes on to ask.

Udall sent the email to supporters on Sunday; the books close on campaign and other committees' fundraising efforts on Monday, the last night of the year. It's in his best interest to start 2013 with a strong showing in fundraising as he begins to gear up for reelection.

And Udall warns that his reelection could affect the Democratic majority in the Senate, an argument used by a number of Democrats this cycle. President Obama won the state by about six percentage points this year.

"There's lots of work to do, and I intend to make sure that we keep the majority in the Senate in the next round of elections. Conventional wisdom says that won’t be an easy task," he says.