Don't change horses at midstream.
1. Proverb Do not try 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—don't change horses in midstream, as the saying goes.
2. Proverb By extension, do not make major changes to a situation or course of action that is already underway. I'm really not confident in the strength of my essay, but I guess I just have to see this one through at this point. Like they say, don't change horses at midstream.