After initially entering into an agreement with Vice President Frank Underwood, he refuses to instruct his followers to help an amendment to a spending bill pass that would, among other things, raise the age of retirement to 68. Mendoza fears that he will not remain Republican Leader in the Senate if Haas' caucus turns against him, so he informs Frank that they intend to vote the amendment down.
Underwood begins to "poach" votes from Mendoza and the Republicans using legislative bribery to convince Republican senators either to vote for the amendment or to abstain. Once it becomes clear that they would not be able to defeat the amendment if a vote were taken, Haas retaliates by suggesting that they stall voting on the amendment until the next recess by calling the roll very slowly (one name per hour). Frank thwarts this tactic too by taking his rightful place in the chamber as President of the Senate and instructing the clerk to take the roll in a "more expeditious" manner.
Knowing that they don't have enough votes to defeat the amendment, Mendoza instructs the Republican senators to leave the chamber so that a quorum for business will not be present. This tactic is successful until the Senate Minority Leader motions to compel the attendance of absent senators. The motion is agreed to by all the present senators (expect for Mendoza and two fellow Republican senators) and Frank authorizes the sergeant-at-arms to arrest any absent senators and bring them back to the Senate chamber. Mendoza and the remaining Republican senators leave and, as a symbolic gesture to emphasize their unwillingness to let the amendment pass, he and his "six best actors" are subsequently handcuffed and carried into the chamber by Capitol Police "under protest".
As a last resort, Haas threatens to "filibuster the main bill". However, Frank and Mendoza struck a deal whereby the passage of the amendment constitutes the passage of the main bill. The Senate passes the amendment and therefore the bill, and it is sent back to the House with the Senate's amendments.
He appears later in the season, alongside Hector Mendoza once again. Both are seen addressing a crowd of reporters on the situation regarding the Chinese money laundering scandal surrounding President Garrett Walker and call for his impeachment. They are later confronted by Senator Michael Kern, who by Frank's subtle prompting, informs them he has five Democratic votes in the Senate willing to impeach, including himself. Curtis asks if he'd want to join them in appearing before the media and he responds by saying, "the country could use some bipartisanship in action."