Can a president withdraw a candidate for the Supreme Court before he or she is confirmed by the senate. For instance, could Donald Trump (in theory) withdraw his support and nomination of Neil Gorsuch now?
Yes, he can. For example, John Roberts was withdrawn as a replacement for Sandra Day O'Connor before he was confirmed in order to nominate him as Chief Justice. Based on the way the hearings were going and the fact that he was confirmed as Chief Justice, he would have been confirmed. Other examples are: Lyndon Johnson withdrew the nomination of Abe Fortas (at Fortas' request) and George W. Bush withdrew the nomination of Harriet Miers.
Lyndon B. Johnson nominated Abe Fortas, then an associate justice, for Chief Justice. Fortas would have succeeded Earl Warren, who had decided to retire. Controversy ensued regarding Fortas's extrajudicial activities, and at Fortas's request, Johnson withdrew the nomination prior to a vote of the full Senate.