Barack Obama Sings Sweet Home Chicago -- I've Got a BAD CASE of The Blues

Barack Obama once again proved he's the most qualified candidate in the field ... TO SING ... 'cause El Presidente once again flexed his voice during an impromptu blues session with B.B. King -- and it was AWESOME!

It all went down at an event at the White House last night ... when Obama went to thank King and Mick Jagger for performing ... and Buddy Guy talked Obama into joining the gang on stage.

Then -- the guy who impressed with Al Green's "Let's Stay Together" -- began singing along to "Sweet Home Chicago" ... and KILLED IT.

And as the band played him off, Barack and Michelle made their exit ... like frickin' rockstars.

The last time Obama sang​, Mitt Romney attempted to counter​ with a rendition of "America the Beautiful" ... and sucked.

So, we gotta ask ...