  Full control of molecular interactions, including reactive losses, would open
new frontiers in quantum science. Here, we demonstrate extreme tunability of
chemical reaction rates by using an external electric field to shift excited
collision channels of ultracold molecules into degeneracy with the initial
collision channel. In this situation, resonant dipolar interactions mix the
channels at long range, dramatically altering the intermolecular potential. We
prepare fermionic potassium-rubidium (KRb) molecules in their first excited
rotational state and observe a three orders-of-magnitude modulation of the
chemical reaction rate as we tune the electric field strength by a few percent
across resonance. In a quasi-two-dimensional geometry, we accurately determine
the contributions from the three lowest angular momentum projections of the
collisions. Using the resonant features, we shield the molecules from loss and
suppress the reaction rate by up to an order of magnitude below the background
value, realizing a long-lived sample of polar molecules in large electric
fields.
