President Obama joined Sen. Michael Bennet (D-Colo.) in a "tele-town hall" meeting Tuesday night, which was a last-minute add to the president's schedule.
Obama told listeners to ignore the negative ads against Bennet and vote for him next Tuesday, according to the pool report.
The president was supporting his chosen candidate in the race. Bennet is facing former Colorado House Speaker Andrew Romanoff (D) in the Democratic primary. Both candidates have released ads attacking one another.
Obama praised Bennet on the call.
"Michael has been as good of a senator as I expected him to be," the president said. He added: "I know there have been a lot of negative ads running against Michael in the last few weeks, which is sort of politics as usual. But when he came to Washington, he came to get things done and not just play the usual political games."
Romanoff released an ad trying tie to Bennet's voting record to campaign donations from the oil and banking industries. And Bennet has an ad pointing out that while Romanoff's campaign has not accepted political action committee money, Romanoff had run a PAC for 10 years.
The Senate primary pits Obama and former President Clinton against one another. Last month, Clinton endorsed Romanoff and penned a fundraising e-mail for him even though the White House had already weighed in on behalf of Bennet.
Obama was in Colorado in February to fundraise for Bennet.
Bennet is trying to avoid becoming the latest sitting senator to lose a primary. Earlier this year, Sen. Arlen Specter (D-Pa.) lost his primary challenge to Rep. Joe Sestak. And, in Utah, Sen. Bob Bennett (R) lost out on his party's endorsement at the state convention.
The White House has a mixed record in Senate primaries this campaign season. It backed Specter in Pennsylvania and Sen. Blanche Lincoln (D) in Arkansas, where the incumbent barely survived a primary challenge from Lt. Gov. Bill Halter.
— Shane D'Aprile contributed to this post.