The president signed an executive order addressing unsubstantiated claims of fraud Tuesday.
The Illinois attorney general and others are responding.
Legal action is already in the works.
The Illinois attorney general calls the president's executive order "blatantly unconstitutional" and vows to fight it. Other voting rights organizations say the president is trying to address a problem that doesn't exist.
It may have been the stroke of a pen heard around the election world in the U.S., but the president's signature on his most recent executive order may just be the sign of another legal battle brewing over presidential authority.
"There have been three executive orders at least that have, you know, in the past tried to regulate elections. They have all been blocked by courts. We expect this one will be, as well," said Kevin Fee, legal director of the American Civil Liberties Union of Illinois.
The executive order seeks to create a list of confirmed U.S. citizens eligible to vote in each state and to ban the postal service from sending ballots from voters not on that approved list.
"I think this will help a lot with elections," Trump said.
The president repeated claims of widespread voter fraud to justify the order, but provided no proof.
A White House spokeswoman doubled down Wednesday in a statement that reads in part: "The President will do everything in his power to defend the safety and security of American elections and to ensure that only American citizens are voting in them."
The League of Women Voters of Illinois is pushing back against that narrative.
"There is no demonstrated fraud activity of any significance that would cause them to do something like this. And so, it's just more false information and an unnecessary action," said Kathy Cortez, with the League of Women Voters of Illinois.
The president's criticism of mail-in ballots comes as he recently used it himself to vote in a special election in Florida because he couldn't be there.
"The American people should be entitled to the same mail-in balloting that the president himself has availed himself off. I mean, this is something that is obviously useful to the American people if the president himself is using it. It's obvious that people need it; people rely upon it," Fee said.
There are concerns the order could disenfranchise many voters. And legal experts say the president has no constitutional authority over elections.
"It's the state's role to determine who should vote, the manner of voting, to make sure that there's not double counting, which in Illinois they do a very good job of," Cortez said.