Colorado 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) raised $1.1 million in the fourth quarter and has $4.7 million cash on hand for his reelection campaign.

Though Republicans believe Udall is vulnerable for reelection, the nearly $5 million he has in the bank sets him up comfortably for a fight.

This past week, however, new emails emerged between Udall’s staff and local officials that Republicans say indicate the senator’s office pressured officials to clarify a public tally of Coloradans who lost their insurance under ObamaCare, and to reduce the 250,000 number.

The state's insurance commissioner, however, has said there was nothing unusual about the exchanges.

Republicans have hammered vulnerable Democrats on reports that people have lost their insurance under the healthcare law, a development that has revealed President Obama’s election-year declaration that anyone could keep their plans under ObamaCare if they liked them to be erroneous.

Republicans believe that controversy could damage Udall going forward. One of his Republican opponents in the Senate race, state Rep. Amy Stephens, accused his staff of “bullying.”

"This kind of bullying and pressure of state employees is just not appropriate," Stephens said during a Monday hearing, according to the Colorado Springs Gazette.

Four Republicans have launched bids to take on Udall: Stephens, 2010 Senate nominee Ken Buck and state Sens. Owen Hill and Randy Baumgardner. None of the Republicans have yet to release their fourth quarter tallies.

—This piece was updated at 4:35 p.m.