Colorado Secretary of State Jena Griswold threatened to refer President Donald Trump for prosecution in any suspected cases of double-voting in the state, Fox News reports.

Her Twitter post came days after Trump discussed testing the mail-in voting system in North Carolina. He told people to vote by mail and then to go and try and vote in person.

“In Colorado, we take double voting seriously and refer all suspected cases for legal enforcement,” Griswold posted on Twitter. “If it makes sense, I will include @realDonaldTrump in the referral for prosecution. He may not have presidential immunity anymore depending on the election.”

In order to test the mail-in system, Trump said, “So let them send it in and let them go vote, and if their system’s as good as they say it is, then obviously they won’t be able to vote. If it isn’t tabulated, they’ll be able to vote.”

He then told reporters: “You can’t let them take your vote away. These people are playing dirty politics. So if you have an absentee ballot, or as I call it, a solicited ballot, you send it in. But I would check it in any event. Go and follow it and go vote.”

Trump was criticized for suggesting people should vote twice, which is illegal. But the president took to Twitter to defend his comment. He said he wanted voters to double-check that their vote had been counted.

“Based on the massive number of Unsolicited & Solicited Ballots that will be sent to potential Voters for the upcoming 2020 Election, & in order for you to MAKE SURE YOUR VOTE COUNTS & IS COUNTED, SIGN & MAIL IN your Ballot as EARLY as possible,” Trump tweeted Thursday morning. “On Election Day, or Early Voting, go to your Polling Place to see whether or not your Mail In Vote has been Tabulated (Counted),” the president continued. “If it has you will not be able to Vote & the Mail In System worked properly. If it has not been Counted, VOTE (which is a citizen’s right to do).”

Griswold told The Denver Post that Trump could be “partially to blame” if there are double-voting cases.

“It’s important to underline to the president and the U.S. attorney general and anyone who is confused that it’s illegal to double vote,” she told the newspaper. “We have safeguards in place, including signature verification, laws on ballot collection, and checking the participation in other states. If the president is causing people to vote twice, he could be partially to blame and we’ll explore the options if it happens.”