The White House did not get a heads-up about Bill Clinton's endorsement in the Colorado Senate race.

White House spokesman Bill Burton said there was no advance warning that Clinton would endorse Andrew Romanoff in the Democratic primary. The Clinton announcement came in the form of an e-mail to Romanoff supporters that went out Tuesday afternoon.

President Barack Obama supports incumbent Sen. Michael Bennet (D-Colo.) and has fundraised for him.

Burton, speaking to reporters on Air Force One en route to Wisconsin, downplayed the former president's decision to break with Obama on the race.

"It sounds to me like just a difference of opinion in a primary," he said.

When asked again about reaction inside the White House, he said: "There is a lot of things that happen in the White House every day. This was just one of those things where, OK, we have a difference of opinion in this primary."

The primary is August 10. The latest numbers in the race, a Denver Post/SurveyUSA poll taken a few weeks ago, had Bennet leading by 17 points.