The Constitution has two main provisions that govern the selection of presidential electors. First, the Constitution says that each state’s legislature has the authority to determine that state’s manner of choosing its electors. Second, the Constitution gives Congress the power to decide when the electors are chosen, which Congress has done by enacting a federal law designating the Tuesday after the first Monday in November — Election Day.
Proponents of the legislative-appointment theory read too much into the first constitutional provision and forget about the second. Although every state has chosen its electors by popular vote for more than a century, most constitutional experts agree that, under the legislature’s authority to choose the “manner” of appointing electors, a legislature could theoretically decide before Election Day to cancel the popular vote for presidential electors and instead appoint them directly. But Congress’s enactment of a uniform national Election Day under its own constitutional authority — which supersedes any contrary state actions — prohibits the choice of electors from being made based on elections held or laws passed after Election Day.
In other words, under the constitutional timing provision as implemented by federal law, the absolute last day a state legislature could have decided to appoint the state’s presidential electors for this election was November 3, 2020. Once that date passed, the determinative popular votes had all been cast, and therefore the legislature’s authority to change the state’s manner of appointing electors in 2020 passed as well.