Expanding on that "last-ditch chance" a little more, if an objection is raised during the joint session of Congress, there are then votes held in the House and Senate separately to decide whether to reject the electoral votes that are in question. Unless both chambers agree, the electoral votes remain valid and must be tallied.

At least that's my non-expert understanding from reading 3 U.S. Code § 15:

https://www.law.cornell.edu/uscode/text/3/15