Beach Boys singer Bruce Johnston is taking a page out of the Jon Lovitz playbook.

Johnston, currently on tour with the reunited Boys, let loose on President Barack Obama during an autograph signing session, as captured by TMZ.

Americans will be "f**ked" if Barack Obama gets re-elected ... Obama's an a**hole," Johnston says ... adding, "Unless you're interested in never having any money and being socialized."

But Johnston was an equal opportunity disser -- saying, "And who's the Republican a**hole?"

"Our guy isn't any good."