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NFL great Fran Tarkenton says athletes who turn down an invitation to Trump's White House are "disrespectful" and "stupid" ... adding, "Nobody boycotted Obama 'cause he's black, did they?"

Tarkenton -- a self-described "huge Trump guy" -- was out in NYC when we asked his thoughts on NBA star Iman Shumpert saying he'd turn down an invite to 1600 Penn. if the Cavs won the championship during Trump's presidency.

Shumpert's comments came in the wake of LeBron James saying he's on the fence about the idea.

Tarkenton was OUTRAGED -- saying athletes should show the same level of respect to Trump they showed for Obama ... despite their personal feelings.

"If a President of the United States, our #1 guy, invites you to the [White House] and you're gonna boycott it? Because why? It's stupid."