When not to run for the presidency? When you are born in another country. Especially when your parents were not out of the country due to a US mission of some sort, like John McCain. Except if it was an emergency procedure, and it could not be helped that your mother had to give birth to you outside the US.
Given that, I know that legally a person such as Senator Ted Cruz could be president, but the fact is that his mother, an American citizen, was living in Canada when she gave birth to Cruz, and she was not working for the US government. Because of that, Cruz should give up his dreams. Have we not had enough issues with ‘birthers’ on the Republican side? We do not need the Democrat version of them as well, and their attack would actually be supported be a Canadian birth certificate.
Purely for the good of party politics, Cruz should take his name out of the issue, so another may rise.