The decision comes as the Postal Service moves to implement regulations on mail-in ballots that had been on hold pending the Supreme Court's action. The changes could have a significant impact on voters.
Under the proposal, the Postal Service would require verification information for all mail-in ballots. States would be required to collect and report basic voter information, such as names and addresses, which would then be attached to a barcode for each mail ballot.
The information would be submitted through a federal portal. In states that do not comply, the Postal Service would not deliver those ballots.
Critics have questioned both the legality and practicality of the proposal.
"So first of all, we have the fundamental constitutional concern that there is no role for the United States Postal Service in the management and oversight of our elections," said Ed Yohnka of the American Civil Liberties Union of Illinois. "That is reserved for individual states to oversee."
The proposal follows a March executive order from Trump that sought to establish statewide voter citizenship lists. A federal judge had issued an injunction against that effort, but the Supreme Court's ruling now clears the way for it to be implemented as well.
The timing of the changes has also drawn scrutiny. With the election approaching, states such as Illinois are already preparing to send mail ballots to voters who have requested them.
"This is a rushed botched job which grows out of the fever dreams of the president about so-called voter fraud," Yohnka said.
Trump's executive order calls on the administration to create lists of eligible voters and directs the Postal Service to deliver mail ballots only to people on those lists.
Mail voting has long been a target of criticism from Trump, who has claimed it breeds fraud, despite using it himself this month.
The League of Women Voters of Illinois disputes that characterization.
"I know there have been lots of accusations, lots of claims of evidence, but to the best of my knowledge, none has been delivered," said Kathy Cortez of the League of Women Voters of Illinois.
Mail-in voting has grown in popularity, with nearly one-third of ballots cast that way in the 2024 election, according to federal data.
Illinois officials have maintained that the system is secure.
A study released last year by the Brookings Institution found four cases of fraud for every 10 million mail ballots cast.
Cortez said she is concerned about the broader impact of the administration's actions.
"It would appear that the administration is using vote by mail or the ongoing use of vote by mail as a potential way to discredit the results of the upcoming November election in some way," she said.
Some election law experts reacting to the Supreme Court's decision have emphasized that it does not resolve questions about the federal government's authority to interfere with state mail-voting policies.
If the administration takes concrete steps to implement the changes, additional litigation is expected.
The Illinois Attorney General's Office said in a statement, "The U.S. Constitution gives states authority to administer elections, and we remain committed to defending the Constitution and right of all Americans to vote."
Illinois Gov. JB Pritzker issued a statement, saying, "The Supreme Court's decision allowing Donald Trump and the U.S. Postal Service to move forward with this unprecedented attempt to interfere in our elections is deeply troubling. We are weeks away from voting in the midterms and the President is trying to put fear in Americans about the security of our elections, rewrite the rules, and put the federal government between voters and their ballots. Illinois will continue to administer free and fair elections in our state while fighting every attack on voter's rights. Instead of focusing on being President, Donald Trump aspires to be a king. In America, the people of this nation decide the outcome of their government."