Frank Underwood tried to work with him to get a spending bill through the Republican-controlled Senate. Their agreement broke down under pressure from Tea Partier Senator Curtis Haas, and Mendoza ordered the Republican senators to leave the Senate chamber during a quorum call, in order that Frank (who was President of the Senate as part of his role as Vice President) would not be able to take a vote on the bill. However, the Senate Minority Leader moved to compel the attendance of absent senators, which motion was easily passed due to the lack of Republicans in the chamber, and the sergeant-at-arms was instructed to arrest any absent Senators and escort them back to the Senate chamber. Consequently, Mendoza and his fellow Republicans were handcuffed and carried into the chamber "under protest" by Capitol Police so that a quorum would be present. The amendment was then passed by the Senate.
Mendoza meets with Claire Underwood regarding her desire to be appointed as the U.S. Ambassador to the U.N. He claims he will not openly oppose her, but at a later hearing antagonizes her with questions. Claire's reaction, claiming the US Military is 'irrelevant' prevents her from attaining the role. Mendoza later reveals he only antagonized her to see how she reacted under pressure, testing her suitability for the role.
Mendoza is named as a strong contender for the Republican nomination for the 2016 Presidential Election by Bob Birch, but neither confirms or denies this to Frank Underwood. It revealed later in the season that amidst a scandal of accepting undeclared payments for speeches, Mendoza had resigned. He was replaced as Senate Majority Leader by Senator Henry Mitchell.