Bill Clinton was president for two terms. Had he been president for two minutes, he would have earned the lifestime right to be called Mr. President.
Hillary, while Bill was in office, was the first lady. Now, as she runs for president, things have changed. Or, if she wins, that is. The First Lady would get an upgrade, Mrs. President. They could call her Ms. President, but no one is silly enough to think she is not married.
What do we call Bill, then, while she is in office? Do reporters call out, "Mr. and Mrs. President!" or "Mrs. President, Mr. President!" or, in cases that follow as such (where 'first lady' was previously appropriate), "Mrs. President, what does Mr. President think about ---?"
I guess Chelsea will be among the few without a problem. Or Monica.