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Woody Allen didn't vote for Donald Trump ... but, it seems he's got support if the guy ever wants back into showbusiness -- 'cause he's a great professional actor!

The Hollywood legend stopped by Bill Maher 's 'Club Random' podcast and opened up about directing the President of the United States in his 1998 movie celebrity.

Allen shouted out the president's professionalism ... saying he was polite to everyone on the set -- and giving his acting a thumb's up.

If you don't know ... Trump makes a small cameo as himself in the 1998 movie "Celebrity" -- in which he says he plans to buy, then tear down, St. Patrick's Cathedral and put up a "very tall and beautiful building" instead.

Maher tells Allen his comments will offend some people ... and, Woody clarifies that he d

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