Politically, a recommendation for indictment by the FBI would be almost as damaging as an actual indictment. And if the DOJ failed to pursue the recommendation for indictment, then Obama and his administration will be charged with the cover-up. Hence, the constitutional crisis. Something similar happened during Watergate, but the big difference was that the Reps in Congress supported pursuing charges. Today, the Dems will close ranks and blame the FBI for being partisan and interfering in the electoral process.
The reaction of the public remains to be seen,
The President, acting through the AG, has broad discretionary power to convene or not convene a Grand Jury, and, once convened, to present or not present evidence to it.
The FBI, like all components of the Executive Branch, takes orders from the President.
So, no crisis of a constitutional nature could be created in this case, whatever Obama does with an FBI referral.