Sure, it was a nice play on the title ("Casa... Blanca"... White House) but having Victor Laszlo become President of the United States in the last scene strains credibility a little too far. Despite an impassioned monologue in which he tells of being born in a Cleveland tenement and of his parents then immediately immigrating back to Prague, Paul Henreid never quite convinces the viewer that he could have won the Republican nomination so quickly. (And the cheap mock-up of the Rose Garden - a testament to the new wartime rationing - doesn't help either.)
This is not to knock Henried, a fine actor in his own right. Rather, the blame has to fall on Jack Warner. Though the ending appeared neither in the play that the film was based on - or any of the several screenplay drafts - the mogul insisted on inserting it at the very last minute, in a misplaced burst of patriotic fever.
While Casablanca remains a beloved favorite of film aficionados everywhere, this is the classic example of a twist that went a little too far.