A democratic political division is forming at the highest level - as a two-year old fracture has reemerged between former President Clinton and President Obama, this time over a democratically-held senate seat in the toss-up state of Colorado.

Mr. Obama enthusiastically endorsed, and hit the trail with democratic incumbent Senator Michael Bennet, who's highly vulnerable in part because he never won an election. Bennet was appointed to the seat left vacant when Ken Salazar became Obama's Interior Secretary.

At an event for Bennet earlier this year, the president rallied supporters in Denver by saying, "As hard as you worked in 2008, you've got to work harder in 2010" for Bennet. 

But this week, once again, the last Democrat to be president came out diametrically opposed to his successor’s political judgment. Bill Clinton is backing an underdog insurgent to oust the White House backed Bennet in the August 10 Colorado primary.

Andrew Romanoff is the former Colorado State Speaker of the House, and supported President Clinton early on in his 1992 presidential campaign.

"In my view, this is a big boost in a campaign that's been gaining momentum every month," Romanoff said, while the Bennet campaigned chalked the endorsement up to loyalty.

In an e-mail letter that caught the White House off guard, Clinton thanked Romanoff for that endorsement in the 1990s and touted a series of accomplishments by Romanoff.

Clinton wrote, "Andrew brings to this race both an extraordinary record of public service and an extraordinary capacity to lead. I believe that those assets, as well as his deep commitment to Colorado, give him the best chance to hold this seat in November."

Clinton began re-flexing his political muscles recently, and is credited with helping endangered democratic incumbent Blanche Lincoln survive her primary runoff in Arkansas. In contrast, President Obama has gone to bat in the last year for candidates in New Jersey, Virginia and Massachusetts, and they have all been defeated.

Endorsements seldom sway races. But when presidents back opposing candidates in primaries, their disagreement can overshadow the candidates on the ballot, split the party, and undercut its chances of success in the fall.

The White House shrugs off any rift or embarrassment, despite the absence of a Clinton-camp courtesy call before or since the endorsement.

"I was asked if we heard from them prior to that endorsement, and the answer to that was no," White House Press Secretary Robert Gibbs said to reporters Thursday. When asked if he would prefer a heads-up in the future, Gibbs quipped, "Before you guys? Sure."

Carl Cameron contributed to this report.