The certification of all Federal elections is ultimately the duty of Congress... Whenever Presidential Electors present themselves for seating to vote for President, the President of the Senate should reject any Elector not selected by a fair and open process. A process free from corruption and illegal conduct.
See Article 2, Section 1, Clause 2 of the US Constitution:
"Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector."
The state’s authority to choose its own method for appointing electors is not in doubt. However, a few other issues remain unresolved:
When issues concerning the seating of Electors arises the question that presents itself is: Can Congress step in if there is controversy regarding an electors' right to represent a state? Congress has taken such action in the past, and it claimed authority to act to resolve such issues in the Electoral Count Act of 1887 and subsequent measures.
However, some scholars argue that such federal laws impinge on the states’ authority, as outlined in Article II, Section 1, Clause 2. There has been no defining SCOTUS decision on this controversy. Therefore, the US Senate remains by default the agency to decide if State Electors are legitimate and should be seated.
Pres. Trump could prevail upon Sen. McConnell to refuse to seat the Electors from those States whose conduct of their elections was unfair and violated the law.
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If the above occurs the President-Elect still must obtain 270 votes from the remaining Electors to become President.