The states were sovereign governments before the ratification of the Constitution. Their own founding documents established their baseline powers after the Declaration of Independence. They first attempted to unify under the Articles of Confederation.
By ratifying the Constitution of the United States, the several states delegated powers to the federal government. One of those powers was the plenary power of the state legislature to choose the method of selecting Electors to the Electoral College. By ratifying the Constitution, the state agreed to the stipulation that the legislature had sole power to set the process for selecting Electors.
Pennsylvania, and other Democrat governed states, are now reneging on that stipulation.
-PJ
And those same founding documents established the role of the state legislature and executive and the process they had to go through to enact state laws, including election laws.
By ratifying the Constitution, the state agreed to the stipulation that the legislature had sole power to set the process for selecting Electors.
The stipulation is the Legislature, not the Executive nor Judiciary of the state, is the body to choose electors. It still has to be done within the confines of the state constitution.
For example, the US Constitution says the state legislatures control the "time, place and manner" of electing representatives and senators, just like they control the selection of electors. If this gives them the plenary power you say it does the partisan majority could sit down on October 20th and say "Based on early voting I think our party's ahead. Let's end this thing tomorrow."
But of course they can't do that. They have to abide by the election laws they've already passed that spell out the election process.
Can they change those laws? Of course, but not by decree. By the legislative process.
Likewise, there are laws the PA legislature has passed defining the process for choosing electors. They can change those laws but they can't just ignore them.
As tempting as the idea is I think it just a deus ex machina device to avoid the reality that the PA Supreme Court is controlled by Democrats.
If the state legislatures really have this absolute power why haven't any ever exercised it before? You can say it's because we've never had a deadlock but a deadlock isn't a requirement in the US Constitution.