Sen. Chris Dodd (D-Conn.) on Wednesday sent out a fundraising e-mail to his donor list on behalf of his Senate colleague Michael Bennet (D-Colo.), who faces a primary challenge. 

The mailer comes one day after Bennet's primary opponent, former Colorado House Speaker Andrew Romanoff, received the endorsement of former President Bill Clinton. The endorsement drove a wedge between Clinton and President Barack Obama, who has backed Bennet. 

The mailer makes no reference to Romanoff, but touts Bennet's achievements in the Senate and attacks Republicans. 

"Since arriving in the Senate last year, Michael fought along with me to pass health care reform, and he even led the charge for a public option. From reforming Wall St. to protecting our environment, I've been impressed with Michael's commitment to setting partisan games aside and getting to work on solving real problems," Dodd wrote. "But while Michael has been fighting for all of us, his Republican opponents have staked out extreme positions, far outside the mainstream in an attempt to win Tea Party support."

The Connecticut senator's letter could be an indicator that the primary race is heating up. It's already been tinged by Romanoff's claim that the White House invited him to apply for a job so that he would exit the race. 

Clinton's endorsement brought up memories of the 2008 presidential campaign; Romanoff endorsed Hillary Clinton, while Bennet — an appointed senator — backed Obama. 

Dodd endorsed Obama in February 2008 after dropping out of the Democratic presidential primary in January.