KANSAS CITY, Kan. (Reuters) - A trial over a Kansas law that requires proof of U.S citizenship from people registering to vote opened on Tuesday, with critics calling it illegal and backers saying it was a necessary tool to fight fraud.

FILE PHOTO: Kansas Secretary of State Kris Kobach talks about the Kansas voter ID law that he pushed to combat what he believes to be rampant voter fraud in the United States in his office in Topeka, Kansas, U.S., on May 12, 2016. REUTERS/Dave Kaup/File Photo

The lawsuit, filed in February 2016 in the U.S. District Court in Kansas City, Kansas by the American Civil Liberties Union (ACLU), argues that the state law violates the National Voter Registration Act by requiring voters who do not have a driver’s license to show documents such as a birth certificate or U.S. passport for voter registration.

It is one of numerous voter ID laws passed by Republican-led state legislatures in recent years.

Democrats argue that ID laws target voters who typically support the Democratic Party, such as the young and minorities. Proponents such as Kansas Secretary of State Kris Kobach say they prevent voter fraud.

Kobach, chief defendant in the case, said in opening statements that 129 non-U.S. citizens had voted or registered to vote in Kansas since 2000.

“That is the tip of the iceberg,” he said in court.

Evidence will show up to 18,000 other non-citizens have voted in Kansas, Kobach said.

ACLU’s lead attorney, Dale Ho, disputed Kobach’s numbers and said the law targets a small number of people who might seek to vote illegally.

“Enforcing this law is like taking a bazooka to a fly, and the collateral damage has been thousands of voters,” Ho said, referring to that group’s estimate that more than 35,000 citizens were blocked from registering to vote between 2013 and 2016.

Witness testimony in the case, being heard by U.S. District Judge Julie Robinson, is expected to last five or more days.

In May 2016, Robinson temporarily blocked enforcement of the law pending the trial’s outcome. It first went into effect on Jan. 1, 2013.

Kobach, a Republican candidate for Kansas governor, is serving as lead attorney for the state.

He served on a commission appointed by U.S. President Donald Trump to investigate voter fraud. Trump contended millions of people voted illegally in the 2016 presidential election he won. The commission was shut down in January.

Lawmakers in 23 states have imposed new voting restrictions since 2010, according to the Brennan Center for Justice at New York University School of Law.

This year, six states have introduced bills imposing photo identification requirements for voting, and bills have been put forward in Kentucky and New Hampshire to make existing voter identification laws more restrictive, the Center said.