Sen. Josh Hawley of Missouri voted to object to the certification of the ballots of PA electors. A statement from him that was printed in a Missouri newspaper is also available on Twitter. Hawley claims that when the PA legislature authorized mail-in ballots what they did was unconstitutional.
The "Pennsylvania Supreme Court .. changed the rules for when mail-in ballots could be returned ... the Pennsylvania Supreme Court threw out the case on on procedural grounds, in violation of its own precedent. To this day, no court has found the mail-in voting scheme to be constitutional, or even heard the merits of the case."
It is true that they tried to change the rules for when mail-in ballots could be returned, but that is a different case. Hawley is deliberately conflating two different issues. The question about timing of return was addressed by the U.S. Supreme Court. This case is about whether absentee voting is constitutional. (Hawley is being dishonest).
So then, here's a bit of history about absentee voting (voting by mail, or mail-in ballots) in PA. You can read what the PA Constitution says about absentee voting here.
"The Legislature shall provide a manner in which .. qualified electors .. [who] are unable to attend at their proper polling places .. may vote, and for the return and canvass of their votes".
And it gives examples of reasons: "because of illness or physical disability or who will not attend a polling place because of the observance of a religious holiday or who cannot vote because of election day duties, in the case of a county employee".
Sen. Hawley relies upon a restrictive reading of this text where he claims that what the PA legislature did was unconstitutional (that only the listed reasons are valid). There has been a lot of discussion about why that is not the case (speech by Sen. Casey, good newspaper review). It is true that the PA Supreme Court has not actually ruled on this issue.
In October, 2019 (not "last year"), the PA legislature passed, and the governor signed into law Act 77 of 2019 in Pennsylvania (P.L. 552, No. 77). You can read the text here, but like most legislation it is frankly, unreadable. It provides for universal mail-in ballots.
It is noteworthy that the bill was introduced by Republicans and passed with majority support by Republicans. Several previous elections have been run using these procedures.
On Nov. 21 (18 days after the 2020 presidential election) a suit was filed (by U.S. Representative Mike Kelly and others) making the claim of unconstitutionality. The PA Supreme Court ruled against them on Nov. 28, on the grounds that their claim was untimely. You can read the Order here.
"As a remedy, Petitioners sought to invalidate the ballots of the millions of Pennsylvania voters who utilized the mail-in voting procedures established by Act 77 and count only those ballots that Petitioners deem to be “legal votes."
In a concurrence, Justice Wecht wrote
"Unsatisfied with the results of that wager, they would now flip over the table, scattering to the shadows the votes of millions of Pennsylvanians,"
In summary then, Republicans voted to allow mail-in ballots in PA, ran several elections this way without challenging (their own) law, and then filed suit to challenge the process, more than two weeks after their preferred candidate for President lost the election in PA.
Not only did the PA Supreme Court dismiss this case, but it is a fundamental tenet of election law (as promulgated by the U.S. Supreme Court), that authorities may not change the rules after the election. Voters are entitled to rely on the rules established beforehand.
You can read a good post about election law here, and an NPR article about the recent case here.
Sen. Hawley is dishonest, but you knew that. Was this done deliberately by the GOP with the plan to try to overturn the election? Who knows?
What seems clear is that the modern GOP is the party of voter suppression, the party that (arguably) wants to return to Jim Crow, by disenfranchising Black voters.