According to US Legal, it is bribery. See the bold text below:
Election bribery is a criminal offense. The following is an example of a state statute describing election bribery:
Any person, who offers, gives, lends or promises to give or lend, or attempts to procure anything of value or any office or employment or any privilege or immunity to, or for, any elector, or to or for any other person, in order to induce any elector to:
Refrain from going to the polls.
Vote or refrain from voting.
Vote or refrain from voting for or against a particular person.
Vote or refrain from voting for or against a particular referendum; or on account of any elector having done any of the above.
Pursuant to 42 USCS § 1973i, whoever knowingly or willfully pays or offers to pay or accepts payment either for registration to vote or for voting will be fined not more than $10,000 or imprisoned not more than five years, or both. However, this is applicable only to general, special, or primary elections held solely or in part for the purpose of selecting or electing any candidate for the office of President, Vice President, presidential elector, Member of the United States Senate, Member of the United States House of Representatives, Delegate from the District of Columbia, Guam, or the Virgin Islands, or Resident Commissioner of the Commonwealth of Puerto Rico.
Also note that Electors are prohibited from meeting in one central location.
To minimize the chances of corruption, bribery and backroom deals, electors are prohibited from gathering in one central location to cast their ballots. Thus, electors meet in individual state capitals to vote.
Most certainly if bribery were found to cause a change in the election of whom was projected President, the Congress would not certify that result and choose the correct candidate.