Can a presidential candidate withdraw AFTER the election?

My question is this: Say we had another “2000”, ie, a candidate wins the Electoral College but loses the popular vote. This time, the “loser” had several million more votes than the “winner”. The now President-elect feels genuine regret for this and wants his rival to be President, having soundly defeated him vote-wise.

Can he withdraw? If not, would the Speaker of the House take over after both President and Vice-President (after what would surely be a marathon arm-twisting session) withdrew?

Clarification needed: Does the president-elect withdraw before the Electoral College actually meets and votes in December? I ask because most people assume a presidential candidate will win the Electoral College based on the states he has won in the general election. If he withdraws after the general election but before the Electoral College votes, the electors can still vote for someone else.

Prior to the Electoral College meeting, the Electoral College will meet and vote on the president. If the leading candidate in Electoral votes has withdrawn, his electors can decide who they vote for. In theory, it can be anyone eligible for president. In practice, it would be a substitute candidate named by the party leaders, and most likely the vice presidential candidate.

If the electoral college has voted, things get very messy, and there’s no clearcut answer.

Another clarification needed: when does the vice president-elect withdraw?

Similar question from two weeks ago: What if a president changes his mind about being president?

For the purposes of this question, the Electoral College has already voted.

I wonder if the succession rules for resignation of an incumbent President would come into play?

From the other thread:

Before January 20, the president-elect and vice president-elect could send letters to the Secretary of State (who under law receives the official notice of a president’s resignation) and the President of the Senate (who officially tallies and announces the results of the Electoral College vote), saying, “I hereby decline, and will be unable to serve, the office of President (Vice President) of the United States.”

With a vacancy in the presidency on January 20, the Speaker of the House would become president, per the Presidential Succession Act. He or she could then nominate a vice president, to be confirmed by Congress, per the 25th Amendment.

Well if the elected President really wanted the other candidate to be President, he should (a) take office, (b) get his Vice President to resign © appoint the other candidate his Vice President and push to get it through both houses, (d) resign himself.