WASHINGTON (Reuters) - The Republican-controlled U.S. House of Representatives plans to vote on Wednesday on a bill banning non-citizens from voting in federal elections, a rare practice that is already illegal, fueled by Donald Trump's false claims about election fraud.

Less than four months before Trump's Nov. 5 election showdown with Democratic President Joe Biden, House Republican leaders including Speaker Mike Johnson are backing the measure based on claims that people who cross the U.S.-Mexico border illegally could cast ballots in this year's presidential and congressional elections.

On Tuesday, Johnson claimed without evidence that Democrats are encouraging non-citizen voting.

"Many of the Democrats want all of these illegals to participate in our federal elections. They want them to vote," he said at a news conference.

The legislation is likely to be dead on arrival in the Democratic-majority Senate. Democrats have already called the bill a "stunt" aimed at sowing "confusion and distrust" and warned that it constitutes a form of voter suppression.

"For all the hysterical rhetoric ... Republicans have one real purpose here: to continue to erode the confidence of Americans in our election system," said Representative Joe Morelle, top Democrat on the House Administration Committee, which oversees election policy.

Democratic Representative Teresa Leger Fernandez warned the legislation, if enacted, would make it harder for U.S. citizens to vote, including armed services members, Native Americans, minorities and people who live in rural areas.

"It is shameful and unpatriotic when Republicans take aim at voter participation," the New Mexico Democrat said.

Johnson, who relied on Trump's support to hold off a bid to oust him from leadership by Republican hardliner Marjorie Taylor Greene, unveiled plans for the legislation during an April joint press conference with the former president in Florida.

Trump, who continues to repeat false claims that the 2020 election was stolen through massive voter fraud, favors the bill.

Republicans say the legislation would safeguard voting by requiring states to collect proof of U.S. citizenship before registering voters and to purge their voter rolls of non-citizens.

Johnson and other Republicans have also cited data showing non-citizens are registered to vote in some states and point to municipalities including New York City, Washington, D.C., and Montpelier, Vermont, that have allowed resident aliens to vote in some local elections.

Republicans also rely in part on a widely rejected 2014 study that claimed to show a level of noncitizen voting capable of swaying congressional and presidential elections. Trump used the study to support his claim that Democrat Hillary Clinton won the popular vote in the 2016 presidential election because of illegal ballots cast by non-citizens.

"This is a scare tactic," said Wendy Weiser, who directs the democracy program at the Brennan Center for Justice at New York University. "Non-citizens cannot vote in U.S. elections. There are multiple layers of protections in place, including it being a crime under federal and state law."

A Brennan Center study examined 42 jurisdictions, accounting for 23.5 million votes in the 2016 presidential election, and found only 30 incidents of possible non-citizen voting, or 0.0001% of votes cast.

(Reporting by David Morgan; Editing by Scott Malone and Christian Schmollinger)

By David Morgan