DENVER -- U.S. Senate candidates Bob Schaffer and Mark Udall agree they want to debate each other, but haven't settled on how or when.

Schaffer, a former Republican congressman, has suggested seven debates with no moderators. He said that would allow the candidates to frame the issues themselves.

But Udall, a Democratic congressman, has rejected the format. He said Friday that Schaffer's idea wouldn't allow the candidates to answer questions from Colorado voters.

Udall's aides they would start talks with Schaffer's campaign as early as this month and that some debates would be scheduled before Labor Day.

"We're going to debate, whether it's on Bob's suggested schedule or our suggested schedule or a combination," Udall said.

Schaffer could start as soon as Memorial Day week, said Schaffer's campaign manager, Dick Wadhams

"Bob told him, `Look, if you don't want seven, we'll work on the number," Wadhams said. "We're open to the format even."

Udall and Schaffer are running to succeed Republican Wayne Allard, who's retiring. The race is expected to be one of the most competitive in the country this year.