Storm Surge by Michael R. Stern

As Fritz Russell's summer of exploring the past with the portal comes to a close, the president recruits him for a crucial mission - stop a coup that threatens the outcome of a vital election.

With moles inside the government passing crucial information about the portal, Fritz must be cautious about how it's used, and who he can trust. With surprise visits from Ben Franklin, Winston Churchill, and Albert Einstein, Fritz attempts to prevent himself from becoming a pawn in a complex game of chess.

But can Fritz put an end to the conspiracy and prevent checkmate?