The complaint is that it is in violation of the Pennsylvania constitution, not the U.S. constitution. The argument is that the Pennsylvania constitution requires that voting be done in person except for certain specific circumstances outlined in the constitution where absentee voting is permitted.
A big problem here is that the constitution itself does not actually say that. It grants the legislature the authority to determine how elections are done. The constitution does specifically require that the legislature establish provisions to allow for absentee voting for some circumstances.
The first assertion is that the constitution requires in person voting because that is the way Pennsylvania courts have traditionally ruled. Unfortunately, the complaint does not cite any case law to support that assertion, making it rather difficult to evaluate the validity of the claim.
The second assertion is that the legislature previously obtained constitutional amendments before passing laws establishing and expanding absentee voting. Before the law was passed last year to allow no-excuse mail-in voting, there was a bill to put an amendment to the constitution on the ballot to require mail-in voting. That is has to be passed by a second legislative session next year and can then go on the November 2021 election ballot.
The assertion is that this demonstrates that the legislature had acknowledged that a constitutional amendment was required to allow no-excuse mail-in voting. But that appears to be false. It appears that the process used for absentee ballots and the proposed amendment for mail-in ballots was simply to make those forms of mailing be required, not because an amendment is a prerequisite to passing such laws.
And, of course, there is an issue with the timing of this filing. They had more than a year to object to the mail-in voting law prior to the election but chose not to do so until after the results of the election were known. Federal law requires states to conduct elections according to the laws in place on election days precisely for this reason.
If you want to meet the deadline for the guideline of filing a motion upholding the decision of the provisions that were written intently by the general assembly as indisputably constitutionally forbidden in the court of Pennsylvania…YOU’RE TOO LATE…because it juuuuuuuust passssssssed byyyyyyyy!