Neither major partys presidential candidate has officially announced a running mate, but speculation as to how those choices will impact each ticket has already flooded political conversation. But before getting swept away by the deluge, it is important to understand what the vice president can and cannot do, on the campaign trail, and in the White House.

Research in my new book, On Behalf of the President , suggests that the barriers vice presidents face to exerting political influence, in addition to those inherent to their official limitations, are exceedingly high. Across the last three administrations, vice presidents have given fewer public speeches than presidents and first ladies, and within the universe of those speeches, fewer campaign remarks than both. According to data provided on the White House Briefing Room website, in 2015, Joe Biden made 16 public speeches compared to Michelle Obama s 76. Vice presidents also have lower rates of name recognition than presidents and first ladies, and they are generally less popular than presidents and first ladies. Vice presidential favorability also tracks very closely with presidential favorability. In almost every year from 1993 to 2014, average favorability ratings of the president and vice president have either both increased or both decreased. This is not the case for the presidential spouses, who can retain or increase their popularity even when the presidents falters.