"I misspoke back then".
I guess he did it repeatedly then.
And if the top lawmaker in the US has the propensity to misspeak repeatedly on highly scrutinized issues, perhaps he should reconsider whether he has the mental ability to hold down the job.
After all, if a brain surgeon kept saying "My hand slipped" during surgery, you might not fire/jail him, but you sure as hell would move him from the neurosurgery department to something less important.
If you don't believe he is a liar - you have to accept his claim of repeated incompetence/mistakes. And that position is just too important to leave in ths hands of a man who admittedly messes up this much on topics he is working on so intently.