You don't change horses mid-race.
It is dangerous or inadvisable to choose or back a different political figure for an election after the decision has already been made or the position filled. Many people are dissatisfied with the senator's performance but will likely carry his party's support through to the next election—you don't change horses mid-race, as the saying goes.
in midair
Moving through the air. One of their players was able to intercept the ball in midair and the score a touchdown. Look at this great picture I've got of you in midair! And here's one from right when you hit the water.
in midair
in a point high in the air. The planes crashed in midair. Extra fuel was released from the plane in midair.