In order to prove that Obama was not legally entitled to be President, many Republicans alleged that his Birth Certificate was false and that, even though his mother was an American citizen, he actually was born in another country, and therefore not a natural born citizen. They were unable to prove that the Birth Certificate was false, in a court of law, and their lawsuits did not prosper. Nevertheless, to this day, a plurality of Republicans profess the illegitimacy of the Obama presidency.
When faced, however, with the case of Ted Cruz, who actually was born in a foreign country, as proven by his Canadian birth certificate, most of these same "birthers" claim that he is quite eligible, to be President, because his mother was an American citizen, even though he was born abroad.
Of course, any question as to why the double standard would, in their judgement, be playing the race card.