Sen. Mark UdallMark UdallDemocratic primary could upend bid for Colorado seat Picking 2018 candidates pits McConnell vs. GOP groups Gorsuch's critics, running out of arguments, falsely scream 'sexist' MORE (D-Colo.) raised $2 million in his first fundraising quarter since Rep. Cory GardnerCory Scott GardnerRepublicans jockey for position on immigration Bipartisan bill would toughen North Korea sanctions, require Trump's strategy GOP senators push for delay of ObamaCare insurer tax 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.