Part of the letter included the following:
“For every politician — Democrat and Republican — who is tut-tutting that Davis must resign, they are defending a hypocritical standard. Where is the call for the mayor of San Francisco to resign for creating a sanctuary city — resulting in the murder of American citizens by criminal illegal aliens welcomed by his lawlessness?
"Where is the call for President Obama to resign for ignoring and defying our immigration laws, our welfare reform laws, and even his own Obamacare?
“When the mayor of San Francisco and President Obama resign, then we can talk about Kim Davis.
I believe this could be considered a version of the Tu Quoque fallacy, which essentially is an attempt to dismiss an opponent's argument by pointing out that the person or group making the argument has said or acted in a manner which is inconsistent with it. Generally, the problem with this sort of attack is that just because someone has acted hypocritically doesn't necessarily mean their argument is wrong.
A classic example of the Tu Quoque is:
Dad: "John, you shouldn't smoke. It is very bad for your health!"
John: "I don't see how you can tell me not to smoke, when you smoke yourself."
Clearly smoking is bad for you and just because the father is being a hypocrite by smoking himself doesn't effect the truthfulness of his argument. Actually, in this particular example, the hypocrisy could serve to strengthen the fathers argument since he has personal experience dealing with addiction and the damaging effects of smoking.
Going back to the Cruz quote, he seems to be implying that if a politician didn't call for the mayor of San Francisco or President Obama to resign, then their inconsistent call for the enforcement of the rule of law should somehow invalidate their argument here. Just as smoking is bad for John's health regardless of whether his father smokes, the law should govern the nation as opposed to the arbitrary decisions of individual government officials. Whether or not a politician has consistently affirmed this principle doesn't change its rightfulness.