From Mike Allen at Politico:

Retired Gen. Colin Powell, once considered a potential running mate for Sen. John McCain (R-Ariz.), now may endorse his opponent, Sen. Barack Obama (D-Ill.), according to Republican sources. But an air of mystery surrounds Powell’s planned live appearance Sunday on NBC’s “Meet the Press,” and no one is sure what he will say.

Powell’s unassailable national security credentials could sway voters who are vacillating about whether Obama is ready to be commander in chief, and his endorsement of the Illinois senator would make a national security emphasis by McCain in the election’s closing days extremely difficult.

Powell, 71, a professional soldier for 35 years, has advised the last three Republican presidents.

The general’s camp is being coy about what he might or might not say on Sunday. But some McCain advisers suspect, without being sure, that Powell will endorse Obama.

“It’s going to make a lot of news, and certainly be personally embarrassing for McCain,” a McCain official said. “It comes at a time when we need momentum, and it would create momentum against us.”