By Alana Wise and Doina Chiacu

WASHINGTON (Reuters) - Republican lawyers said on Monday it was impossible for the Nov. 8 U.S. presidential election to be "rigged" against Donald Trump, despite such allegations by the Republican nominee in recent days.

Trump, a New York businessman making his first run for public office, has sought to raise fears of a flawed election as he has fallen in opinion polls against Democrat Hillary Clinton.

"Of course there is large scale voter fraud happening on and before election day. Why do Republican leaders deny what is going on? So naive!" Trump said on Twitter on Monday.

Numerous studies have shown that voter fraud in U.S. elections is very rare.

Republican campaign lawyer Chris Ashby said Trump's charges, which were not backed by any evidence, could foment unrest and were “unfounded” and “dangerous.”

"When you say an election is rigged, you’re telling voters, your supporters, their votes do not matter,” Ashby said in an interview. "I think some of Donald Trump’s comments could cause unrest at the polls."

Some Republicans have urged Trump to drop the assertions. Early voting and voting by mail have already begun in many states.

Mark Braden, partner at Baker and Hostetler and former chief counsel for the Republican National Committee, said that any sort of election rigging at the national level “just is impossible,” citing the various systems in place that would make such an endeavor complicated and unfeasible.

“Our system is principally a system based upon each side watching the other side,” Braden said in an interview. “Our system is dependent on local volunteer participation. Our system has worked very well because we have people who get involved in the process and perform these functions.”

Trump has tried to portray Clinton, a former U.S. senator and secretary of state, as a corrupt lifelong politician.

His campaign pounced on the release on Monday of Federal Bureau of Investigation documents that cited an FBI official as saying a senior State Department official sought to pressure the bureau in 2015 to drop its insistence that an email from Clinton's private server contained classified information..

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Clinton's decision to use a private server while secretary of state from 2009 to 2013 has drawn criticism that she was careless with national security.

MELANIA TRUMP SPEAKS OUT

Trump's vote-rigging accusations come after his campaign was damaged by the release of a tape 10 days ago showing him lewdly bragging about kissing and touching women without their permission. A series of women have come forward with allegations about his behavior, but he has denied those accusations.

He has said he did not act on his words, but a series of women have since come forward with allegations about his behavior, which he has denied.

The RealClearPolitics average of national opinion polls shows Clinton currently leading Trump by 6.9 percentage points, at 45.9 percent to 39 percent.

On Monday, Trump's wife, Melania Trump, who has not played an active role in his campaign, spoke publicly for the first time on Monday about the "Access Hollywood" tape.

A former model who has mainly stayed off the campaign trail, Trump told CNN she was surprised by her husband's language and said she thought he had been egged on to "say dirty and bad stuff," calling it "boy talk."

She told Fox News Channel that it was hard for Trump, a former reality television star, to run for office because he had been "in so many shows, so many tapes" over the years.

"Those words, they were offensive to me and they were inappropriate. And he apologized to me," Melania Trump said in an excerpt released by Fox News Channel.

"I accept his apology. And we are moving on," she said.

'NOT GOING TO BE RIGGED'

Trump's vice presidential running mate, Mike Pence, and his campaign manager, Kellyanne Conway, have tried to portray the candidate's comments about vote rigging as being aimed at an unfair news media. But Trump has also pointed to fraud "at many polling places."

The country's top elected Republican, House of Representatives Speaker Paul Ryan, has also tried to counter Trump's message about election fraud. Spokeswoman AshLee Strong said Ryan "is fully confident the states will carry out this election with integrity."

In the traditionally hard-fought state of Ohio, the top elections official, a Republican, said concerns about widespread voter fraud were simply not justified.

"I can reassure Donald Trump: I am in charge of elections in Ohio and they're not going to be rigged, I'll make sure of that," Ohio Secretary of State Jon Husted told CNN.

Clinton campaign manager Robby Mook said Trump's assault on the voting system was an act of desperation.

"He knows he's losing and is trying to blame that on the system. This is what losers do," Mook told reporters.

In a report titled "The Truth About Voter Fraud," the Brennan Center for Justice at New York University School of Law cited voter fraud incident rates between 0.00004 percent and 0.0009 percent.

An August study by the Washington Post found 31 credible cases of impersonation fraud out of more than 1 billion votes cast in elections from 2000 to 2014. Arizona State University studies in 2012 and 2016 found similarly low rates.

A number of Republican-led states, citing the need to prevent voter fraud, have passed laws with stricter ID requirements. But several have been struck down by courts that ruled they were designed to hinder minority voting.

Ashby, the Republican campaign lawyer, said Trump seemed to be conflating isolated instances of voter fraud with widespread election rigging.

"The election is not rigged and it cannot be rigged," he said.

(Additional reporting by Luciana Lopez and Jonathan Allen in New York, and Susan Heavey and Susan Cornwell in Washington; Writing by Doina Chiacu and Roberta Rampton; Editing by Leslie Adler and Peter Cooney)