Well, since they're asking: if they decide to treat the law-abiding citizens of Colorado and Washington as dangerous felons; if they decide to allocate their precious law enforcement powers to persecuting and arresting people for following a state law that they have themselves just passed by clear majorities; if they decide that opposing a near majority of Americans in continuing to prosecute the drug war on marijuana, even when the core of their own supporters want an end to Prohibition, and even when that Prohibition makes no sense ... then we will give them hell.
And it will get personal. The president wasn't just once a pot-smoker, he was a very serious pothead. His own life and career prove that this substance is no more potentially damaging to a human being than alcohol, which is not only legal but marketed to us with abandon. The future coalition he has built - especially its Millennial base - will splinter. Maybe even some libertarian Republicans will seize the issue and champion federalism consistently for a change.
But the main reason the president should instruct the Justice Department that this is not an area for discretionary prosecution is that choosing to focus on pot-prohibition in states that have legalized it defies reason. One of my core arguments for Obama has long been his adoption of what I consider pretty reasonable, if always debatable, policies. He is not an extremist, proposing laws and regulations that are designed to make a cultural point or wage a cultural war; he's a pragmatist, trying to fix existing problems.