From Bob Cesca:

Andrew Revkin from the New York Times writes that it's entirely legal
under the terms of the Clean Water Act and the Oil Pollution Act for
the president to take over the site. In fact, it obligates him to
do so.

(A) If a discharge, or a substantial threat of a discharge,
of oil or a hazardous substance from a vessel, offshore facility, or
onshore facility is of such a size or character as to be a substantial
threat to the public health or welfare of the United States (including
but not limited to fish, shellfish, wildlife, other natural resources,
and the public and private beaches and shorelines of the United States),
the President shall direct all Federal, State, and private actions to
remove the discharge or to mitigate or prevent the threat of the
discharge. (the rest of the Switchboard post)

So let's get going.