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Mark Cuban will challenge Donald Trump in the primaries ... IF he decides to run for President in 2020.

Cuban was profiled on the Fox News Channel show "OBJECTified" Sunday night, and he declared which party he'd choose if he were to run ... he says it'd be Republican.

The Mavericks owner says he's still deciding whether he'll throw his hat in the ring, and that a lot depends on whether his wife is down with the run.