Today a joint session of Congress will count the electoral votes. From CRS:
The joint session does not act on any objections that are made. Instead, the joint session is suspended, the Senate withdraws from the House chamber, and each house meets separately to debate the objection and vote whether, based on the objection, to count the vote or votes in question. Both houses must vote separately to agree to the objection by simple majority. Otherwise, the objection fails and the vote or votes are counted. (3 U.S.C. §15 provides that “the two Houses concurrently may reject the vote or votes.”)
The general grounds for an objection to the counting of an electoral vote or votes would appear from the federal statute and from historical sources to be that such vote was not “regularly given” by an elector, and/or that the elector was not “lawfully certified” according to state statutory procedures. The statutory provision first provides in the negative that “no electoral vote ... regularly given by electors whose appointment has been lawfully certified ... from which but one return has been received shall be rejected” (3 U.S.C. §15). The provision then reiterates for clarity that both houses concurrently may reject a vote when not “so regularly given” by electors “so certified” (3 U.S.C. §15). It should be noted that the word lawfully was expressly inserted by the House in the Senate legislation (S. 9, 49th Congress) before the word certified. Such addition arguably provides an indication that Congress thought it might, as grounds for an objection, question and look into the lawfulness of the certification under state law.
The objection that votes were not “regularly given” may, in practice, subsume the objection that the elector was not “lawfully certified,” because a vote given by one not “lawfully certified” may arguably be other than “regularly given.” Nevertheless, the two objections are not necessarily the same. In the case of the “faithless elector” in 1969, described above, the elector was apparently “lawfully certified” by the state, but the objection raised was that the vote was not “regularly given” by such elector. In the above-described 2005 case, the objection was also based on the grounds that the electoral votes “were not, under all of the known circumstances, regularly given.”