“Precisely because I have not served in uniform, I am somebody who strongly believes I have to earn the trust of men and women in uniform."

A coalition of military groups is planning a nationally televised town-hall-style meeting with the presidential candidates near Fort Hood, Tex., the largest active-duty military installation in the country. But so far, only Senator John McCain of Arizona, the presumptive Republican nominee, has agreed to attend.

CBS has agreed to broadcast the meeting live from 9 to 11 p.m. Eastern time on Monday, Aug. 11. The candidates would face questions directly from an audience of 6,000 people, made up of veterans, service members and military families from the base.

Senator Barack Obama of Illinois, the presumptive Democratic nominee, has not agreed to participate.

Carissa Picard, managing director of the Fort Hood Presidential Town Hall Consortium, said she had suggested Aug. 11 and asked the campaign to suggest other dates if that was not convenient, but after several conversations she had not been able to work anything out.

“I’m having extreme difficulty getting the Obama campaign to commit to this event, and we do not understand why,” said Ms. Picard, whose husband is deployed in Iraq. “We made it very clear to them that if they would commit to the event, we would work with them on dates.”

“Senator Obama strongly supports America’s veterans and military families and has worked hard on their behalf in the Senate,” said Phillip Carter, director of Mr. Obama’s veterans effort and an Iraq war veteran. “While we unfortunately had a previously scheduled commitment on the date proposed, Senator Obama looks forward to continuing the dialogue he’s been having throughout the country with veterans on how we can better serve our men and women in uniform as they serve us.”