Donald Trump could be considering a lifelong Democrat as the No. 2 on the Republican presidential ticket: retired Lt. Gen. Michael Flynn, who once headed up the Defense Intelligence Agency and has informally advised Trump on foreign policy during the billionaire's primary campaign.

The New York Post first reported Flynn's formal vetting earlier this week. In an interview, the 57-year-old former general did not deny that he was going through the process.

"I have been honored to serve my country for the past three decades and look forward to serving in other ways now that I am retired from the U.S. Army," he told the paper. "I've been a soldier too long to refuse to entertain any request from a potential commander in chief."

According to a Washington Post report, Trump's consideration of the top military figure -- who also happens to be a vocal critic of President Obama and a proponent of stronger ties with Russia -- is part of an emerging belief that the GOP ticket could best be served with another Washington outsider.

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"Trump-Flynn. I've heard him say that, kind of test out the sound of it," one person familiar with the vetting process told the Post. "I think it's a brand he finds appealing."

Trump has hinted before that people with military backgrounds were under consideration.

"I like the generals. I like the concept of the generals. We're thinking about -- actually there are two of them that are under consideration," Trump told Fox News Wednesday.

Trump has long held that he would tap a running mate with experience in Washington's legislative circles.

"I think I'll probably go the political route, somebody that can help me with legislation and somebody that can help me get things passed and somebody that's been friends with the senators and the congressman and all," Trump told MSNBC in May.

But after a tense visit with Capitol Hill Republicans earlier this week, the Washington Post reports that Trump has become less convinced that he would need a GOP insider to win in November.

Trump is also considering other career politicians in his veepstakes, including former House Speaker Newt Gingrich, New Jersey Gov. Chris Christie and others.