** UPDATE BELOW **
Jake Tapper at ABC News notes:
On Monday, Memorial Day, Joint Chiefs Chairman Admiral Mike Mullen wrote an open letter to troops in uniform that "the U.S. military must remain apolitical at all times. It is and must always be a neutral instrument of the state, no matter which party holds sway."
"The only things we should be wearing on our sleeves are our military insignia," Mullen wrote.
Three days later, Sen. John McCain, R-Ariz, sent a fundraising solicitation using an image of him and Gen. David Petraeus.
"Something is wrong with your judgment when you want to sit down unconditionally with Raul Castro and Iran's President, Mahmoud Ahmadinejad, but you don't take the opportunity to sit down with General Petraeus and learn about the situation in Iraq firsthand," the letter reads. "My friends, this is not the 'change' we need in our next president."
Petraeus' spokesman told ABC, "By no means does the use of his photo mean he has endorsed anybody. He has not. He won't. He remains apolitical." Does Petraeus object to the use of his photo? "He has no comment on that one way or another," the spokesman said.
A sample of the reporting on Mullen's letter:
The highest-ranking U.S. military officer has written an unusual open letter to all those in uniform, warning them to stay out of politics as the United States approaches a presidential election in which the wars in Iraq and Afghanistan will be a central, and certainly divisive, issue.
"The U.S. military must remain apolitical at all times," wrote Admiral Mike Mullen, chairman of the Joint Chiefs of Staff. "It is and must always be a neutral instrument of the state, no matter which party holds sway." ...
The statement to the armed forces is the first essay for the journal Mullen has written as chairman of the Joint Chiefs, and veteran officers said they could not remember when a similar "all-hands" letter had been issued to remind military personnel to remain outside, if not above, contentious political debate.
The essay can be seen as a reflection of the deep concern among senior officers that the U.S. military, which is paying the highest price in carrying out national security policy, may be drawn into politicking this year.
UPDATE: McCain said at a press conference Friday that his campaign shouldn't have used the image of Petraeus and pledged it wouldn't happen again. Watch it: