WASHINGTON - President Donald Trump pressed his unproven claims of voter fraud on Wednesday at the first meeting of a commission he created to study the issue, and implied states that didn't share voter data with the panel had something to hide.

The commission assembled against the backdrop of a series of federal lawsuits and the refusal of many state governments to hand over information about their voters.

In addition, the panel faced lacerating criticism from Democrats and voting rights groups that argued it could be a vehicle to prevent lawful voters from getting to the polls.

Despite evidence that voter fraud is rare in U.S. elections, Trump alluded to his campaign claim that millions of illegal immigrants voted in the November 2016 election.

"Every time voter fraud occurs it cancels out the vote of a lawful citizen," he told the commission. "Any form of illegal or fraudulent voting, whether by non-citizens or the deceased, and any form ofvoter suppression or intimidation must be stopped."

State officials from both parties and election experts widely agree that such voter fraud is rare.

Trump won the state-by-state Electoral College tally that decides the presidential race, but he lost the popular vote by nearly 3 million votes to his Democratic opponent, Hillary Clinton.

He established the Presidential Advisory Commission on Election Integrity in May and assigned Vice President Mike Pence to lead it.

Civil rights groups and Democratic lawmakers have said the commission could lead to new ID requirements and other measures making it harder to vote.

Shortly before the commission assembled, members of the Democratic National Committee’s Commission to Protect American Democracy from the Trump Administration held a news conference to criticize the panel and vow to ensure it does not undermine voting rights.

"I can tell you, President Trump, that we will be watching your commission," said U.S. Representative Terri Sewell of Alabama, an African-American whose district includes the civil rights landmarks Selma, Montgomery and Birmingham, where, she said, "people died, bled and fought for the right to vote."

The panel ran headlong into controversy last month when its vice chair, Kris Kobach, the Republican secretary of state for Kansas and an advocate of tougher laws on immigration and voter identification, asked states to turn over voter information.

The data requested by Kobach included names, the last four digits of Social Security numbers, addresses, birth dates, political affiliation, felony convictions and voting histories.

More than 20 states refused outright and others said they needed to study whether they could provide the data.

Trump questioned the states' refusal, saying, "If any state does not want to share this information, one has to wonder what they're worried about? ... There's something, there always is."

Before the meeting Kobach dismissed his critics' concerns that the panel would advocate votersuppression.

"The commission is not set up to prove or disprove President Trump's claim. The commission is simply to put facts on the table," he said in an interview with CNN.

Several federal lawsuits have been filed against the voting commission. The American Civil Liberties Union's suit alleged the panel failed to comply with federal transparency laws, the NAACP's Legal Defense Fund alleged the commission was formed with the intent to discriminate againstvoters of color. Other lawsuits, including one from the Electronic Privacy Information Center, raised concerns about Americans' privacy.