By Bryan Bender and Matt Viser , Globe Staff

WASHINGTON -- President Obama has been forced to cancel several debate practice sessions with Senator John F. Kerry, whom he tapped to play stand-in for Republican challenger Mitt Romney, leaving little time for them to spar ahead of next week’s first presidential debate.

The Massachusetts senator has held at least one mock debate with Obama to help him prepare for the first matchup against the former Massachusetts governor to be held Oct. 3 in Denver, but several others were cancelled due to the the president’s busy schedule, said deputy Obama campaign manager Stephanie Cutter.

Keeping with a decision at Obama headquarters in Chicago to keep the details of the debate preparations secret, Cutter declined to say how many sessions were cancelled and Kerry declined to be interviewed about how he has been boning up on Romney’s debating style.

But the Obama team appeared Thursday to go out its way to lower expectations for the trio of one-on-one debates that the president will hold with Romney next month.

“I will just take this opportunity to say that Mitt Romney on the other hand has been preparing earlier and with more focus than any presidential candidate in modern history -- not John F. Kennedy, not President Bill Clinton, not President George Bush, not Ronald Reagan has prepared as much as he has,” Jen Psaki, a White House spokeswoman, told reporters on Air Force One, according to the press pool report. “So there’s no question that he will have a lead on how prepared he is.”

She did say that the president has “been doing some studying” but cited his travel schedule, unfolding events in Middle East, and “just the constraints of governing” as preventing the president from focusing more time on it.

The three scheduled debates between Obama and Romney are seen as one of the former Massachusetts governor’s last big chances to open a lead against the president, who is up in most polls.

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