In Season 3 Episode 8 of House of Cards, during Frank's "breaking the fourth wall" instance, he said:
that Hector Mendoza was replaced as the Senate majority leader because he declared a couple of paid speeches as income?
What does that mean?
Quite the opposite - he didn't declare them, which basically means that he lied in his taxes.
From HoD wiki:
It revealed later in the season that amidst a scandal of accepting undeclared payments for speeches, Mendoza had resigned.
According to what Frank said in this episode this came out because a document "leaked".