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.) has raised $1.5 million in the first quarter of 2013, giving him an impressive war chest early in the election cycle.

Udall has $2.5 million in cash on hand, according to spokesman Mike Saccone, who disclosed Udall’s fundraising haul. Senate candidates must report their first-quarter fundraising numbers to the Federal Election Commission later this month.

Udall has yet to draw a well-known Republican challenger, and his fundraising could give potential opponents something to think about.

Senate Democrats must defend 21 seats this cycle, including five belonging to lawmakers who have announced their intention to retire at the end of 2014. They also must defend Secretary of State John KerryJohn Forbes KerryBreitbart editor: Biden's son inked deal with Chinese government days after vice president’s trip State lawmakers pushing for carbon taxes aimed at the poor How America reached a 'What do you expect us to do' foreign policy MORE’s (D-Mass.) former seat, which is being held on an interim basis by Sen. Mo Cowan (D-Mass.).