The Synecdoche, New York actress will replace Julianne Moore in the role after she was forced to bow out due to a scheduling conflict with The Kids Are All Right.
The HBO Films/BBC Films project is writer Morgan's third Tony Blair collaboration with Michael Sheen following The Deal and The Queen. The Special Relationship focuses on Blair's relationship with President Bill Clinton during the early years of his premiership.
Dennis Quaid will play Clinton, while Helen McCrory reprises her role as Cherie Blair. Wimbledon's Richard Loncraine is helming the movie, stepping in for Morgan after he dropped out of directing the film last month.