Sen. Mark UdallMark Emery UdallSenate GOP rejects Trump’s call to go big on gun legislation Democratic primary could upend bid for Colorado seat Picking 2018 candidates pits McConnell vs. GOP groups MORE (D-Colo.) raised $2 million in his first fundraising quarter since Rep. Cory GardnerCory Scott GardnerSenate moving ahead with border bill, despite Trump 13 GOP senators ask administration to pause separation of immigrant families Sessions floats federal law that would protect states that decriminalize marijuana MORE (R-Colo.) jumped in the race against him.

The strong fundraising quarter boosts Udall's cash on hand to $6 million.

Gardner has yet to announce his fundraising totals. He had $800,000 in the bank as of the end of 2013 — but that was before his switch to the Senate race and GOP party leaders' rallying around him.

Polls have found a tight race between the two.