Birtherism isn't all that bad to the liberal media when a rising conservative star may be the target. Just ask the Washington Post and the New York Times, two liberal papers that devoted serious attention to the question of whether Cruz might be constitutionally ineligible for the presidency.
Post staffers Ed O’Keefe and Aaron Blake devoted an article to the matter in the May 7 paper's Style section: the question of Cruz’s eligibility for the presidency. He was born in Canada, but had an American mother, thus making him eligible for 2016, but O'Keefe and Blake glommed on to the fact that the hypothetical objection that one must be born on American soil to be "natural born" has never been definitively adjudicated. This isn't isolated to the Washington Post.