WASHINGTON (Reuters) - U.S. 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 did not share voter data with the panel had something to hide.

The panel has faced lacerating criticism from Democrats and voting rights groups that argued it could be a vehicle for changes that would make it harder for lawful voters to cast ballots - and by a number of state governments reluctant to hand over information about their voters to the White House.

Despite evidence from state officials from both parties that voter fraud is rare in U.S. elections, Trump has claimed 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 of voter suppression or intimidation must be stopped.”

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. The panel will next gather in September, and hold at least four more meetings over nine months.

Some members of the panel emphasized that even small numbers of illegal votes could affect the outcome of elections.

Bill Gardner, a Democrat who has been involved in almost 500 vote recounts in New Hampshire since becoming its secretary of state in 1976, said he has seen 11 end in a tie, 32 decided by one vote, and 202 by fewer than 10 votes.

“I am a witness that every vote matters, and it doesn’t need to be massive voter fraud to sway the outcome,” Gardner said.

Alan King, a Democrat who is a probate judge from Jefferson County in Alabama, said he has not seen evidence of voter fraud - but said local governments need help from Congress to keep up with changing technology.

“We can talk about elections a lot, but if people can’t vote because the machines don’t work, we’ve got a massive, massive problem,” said King.

‘WE WILL BE WATCHING’

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

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“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.

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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.”

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