Don't change horses in midstream
Don't change your leader or your basic position when part-way through a campaign or a project .
From an 1864 speech by Abraham Lincoln, in reply to Delegation from the National Union League who were urging him to be their presidential candidate. 'An old Dutch farmer, who remarked to a companion once that it was not best to swap horses when crossing streams."
See also: the List of Proverbs.