Alexandria Ocasio-Cortez believes in her heart of hearts Bernie Sanders is the guy to beat Trump -- that said ... she'll support Joe Biden if he's the one who takes on Trump.

We got the Congresswoman in D.C. Wednesday -- after Biden cleaned up on Super Tuesday -- and asked her straight-up ... will she support the guy if he wins the Democratic nomination come July, despite being a Sanders supporter and seemingly immovable.

Watch ... AOC says she would, but might need to hold her nose a little. Fact is ... she says she'll always support the party, but notes JB is a thing of the past -- and Bernie is the future.

For her it's pretty simple -- AOC says Bernie's message and platform is very clear and resonates with working-class people. Meanwhile, Biden's platform seems to just be ... beat Trump? That's not good enough for her, and she thinks many feel the same.