Also new to the cast are Scott Speedman and Gil Bellows, according to Deadline.
The political drama, from writer-director Ricky Tollman, follows Platt as a young journalist tracking a political scandal. Also starring are Damian Lewis as a volatile politician and Mena Massoud and Nina Dobrev as his political aides.
Ehle, who won Tony Awards for her performances in The Real Thing and The Coast of Utopia, appeared on Broadway last year in Oslo, earning a Tony nomination for her leading performance. Her screen credits include Zero Dark Thirty and A Quiet Passion.