Leahy is one of the most disliked members of the Senate, on both sides of the aisle, and not trusted, the latter because his word is worthless. He also is rated near the bottom by polls of Senate staffers. Not a pretty picture. Thus calling him a liar has a rather generic rather than a specific meaning. It describes one of his many bad habits.
He's a shanty Irish #$#%#^@^@@@#&!