There's a phrase for this......
October surprise - Wikipedia
In American
political jargon, an
October surprise is a news event deliberately created or timed or sometimes occurring spontaneously to influence the outcome of an election, particularly one for the
U.S. presidency. The reference to the month of October is because
the date for national elections (as well as many state and local elections) is in early November. Therefore, events that take place in late October have greater potential to influence the decisions of prospective voters.
Since the
1972 presidential election (when it came into use), the term "October surprise" has been used preemptively during the campaign season by partisans of one side to discredit late-campaign news by the other side.