I initially understood "defecting from the slate" as "defecting from the plan". And then I checked out "slate" and found its definition is: a list of candidates, officers, etc., to be considered for nomination, appointment, election, or the like.
It seems neither the defintion nor my "plan" is very suitable here.
What does "slate" mean then?
After Hawaii cast its four votes for Mr. Biden, he finished with 306 Electoral College votes, with no electors defecting from the slate.
The vote follows six weeks of unprecedented efforts by Mr. Trump to intervene in the electoral process and change the outcome of an election he lost by about seven million votes. He was joined by many Republicans who supported his unfounded claims of voter fraud, including 126 party members and 18 state attorneys general who supported a case before the Supreme Court that legal experts said had no merit. The court rejected the case on Friday.