It IS unconstitutional! Manifestly. The rules for each state election MUST be made by that state's legislature. For the Congress to usurp that function is patently unconstitutional.
The Democrats - in this as in many things - rely on one major shortcoming in the Federal court system: The Supreme Court cannot simply issue an advisory opinion on the Constitutionality of a law. In order for the law to "become" unconstitutional it is necessary for a Plaintiff with legal standing to file a complaint in Federal District Court, lose the case there and appeal it to the Circuit Court of Appeals, lose there, and go to the Supreme Court. It is quite likely that by the time the Supreme Court even SEES the case, it will be 2024.
Leftists are evil.