Gallagher has always been a Dem; he just ran as a Repubican to get money to campaign on.
For the 10th time or so, the counting of Electoral Votes takes place AFTER the new Congress is seated.
There may be a Dem majority at that time, but a majority prior to that time is irrelevant to the Electoral Votes.
Further, the entire Democrat caucus would not sign onto this. It would take only a few such seats to stop it, unless the GOP is smashed in November with a 50 seat deficit, which is pretty unlikely given how many seats are safe.