This^, Senator Paul believed there were issues, but requested the Senate deal with it Constitutionally and send it back to the states. They would not do it, and the only one with the real power to send it back otherwise was Pence, who proved to be a traitor. Senator Paul also is a big states' rights advocate, so it would have been a violation of principle (both states' rights and the principle of innocent until proven guilty) to not vote to certify. Remember, Trump has never been in doubt of Dr. Paul, and they never had a falling out.