President Donald Trump cast doubt about the neutrality of a commission he created to investigate “election integrity,” tweeting that it was a probe focused on voter fraud.

Critics have warned that the creation of the Presidential Advisory Commission on Election Integrity is just a way of looking for voter fraud, which several previous probes have found is not a widespread problem. Trump himself confirmed those fears when he labeled the probe a “voter fraud panel” in a tweet Saturday morning and suggested election officials ― including at least one Republican ― were concealing information about illegal voters.

Numerous states are refusing to give information to the very distinguished VOTER FRAUD PANEL. What are they trying to hide? — Donald J. Trump (@realDonaldTrump) July 1, 2017

The White House has sought to present the panel, created in May, as a bipartisan, neutral effort to investigate confidence in elections. In a conference call mapping out logistics for the commission this week, Vice President Mike Pence and Kansas Secretary of State Kris Kobach (R), who are leading the commission, told commissioners they were open to whatever the commission would find and didn’t want anyone to have preconceived notions about what its conclusions would be. Appearing on MSNBC Friday, Kobach, who has a history of stoking fears about voter fraud, said the purpose of the panel wasn’t to confirm Trump’s unsubstantiated claim that millions voted illegally in the 2016 election. Kobach said Friday “we may never know” if that’s true.

Trump’s revealing tweet came after several states refused to comply with the commission’s request for voter information, saying they would not assist the commission in painting an inaccurate picture of fraud. Some states, including Kobach’s Kansas, have said they are legally prohibited from providing the commission with some of the sensitive information requested, such as the last four digits of voters’ Social Security numbers.

Kristen Clarke, the president of the Lawyers’ Committee for Civil Rights Under Law, immediately seized on Trump’s tweet to urge states not to comply with the commission’s work, which she called a “sham.”

We continue to urge states to refuse ANY participation in this sham Commission. It's a threat to democracy & vehicle for #VoterSuppression. https://t.co/zJP1hQrob5 — Kristen Clarke (@KristenClarkeJD) July 1, 2017

Earlier this week, voting advocacy groups raised alarm after Trump appointed Hans von Spakovsky, a former Justice Department lawyer, to the commission. Von Spakovsky has long suggested widespread fraud is a problem and advocated more restrictive voting measures.

Trump’s Saturday tweet was the latest example of the president undermining the work of the White House on Twitter. After White House press secretary Sean Spicer insisted Trump’s executive order on immigration didn’t constitute a “travel ban” earlier this year, Trump called it a travel ban in a tweet. The U.S. Court of Appeals for the 9th Circuit cited Trump’s tweet in a June ruling saying the ban was unconstitutional.