swear (that) black is white

To swear, believe, or act like what's true is the opposite of the real situation, especially in order to get what one wants or to advance one's agenda. The government can't keep swearing black is white when it comes to these real estate figures, or else we're going to head into the same economic difficulty we were in ten years ago. The president could swear that black is white, and he would still have people lining up to agree with everything he says.

swear black is white

vigorously maintain anything, however unlikely, in order to get what you want.