(Adds latest Trump tweet, details on Nevada lawsuit, political context, Republican congressman)
By Andy Sullivan
WASHINGTON, Nov 5 (Reuters) - Democrat Joe Biden edged closer to victory over Donald Trump for the U.S. presidency on Thursday as election officials tallied votes in the handful of states that will decide the outcome and the Republican president pursued a litigation strategy.
Trump, who during the long and rancorous campaign attacked the integrity of the U.S. voting system, has alleged voting fraud without providing evidence, filed lawsuits and called for at least one state recount.
His campaign's latest move was a lawsuit expected to be announced later on Thursday alleging voting fraud in Nevada, one of the pivotal states where he narrowly trails Biden.
Some legal experts called the challenges a long shot unlikely to affect the eventual outcome of the election.
As counting continued two days after Election Day, slowed by large numbers of mail-in ballots amid the coronavirus pandemic, Biden was leading in Wisconsin, Nevada and Arizona and closing in on Trump in Georgia and Pennsylvania.
Multiple Trump lawsuits and a recount request would have to succeed and find in some cases tens of thousands of invalid ballots to reverse the result if Biden does prevail.
Some of the outstanding votes in Georgia and Pennsylvania were clustered in places expected to lean Democratic - like the Atlanta and Philadelphia areas.
In Georgia's Fulton County, which includes most of Atlanta, officials said they expected to finish vote tallying on Thursday morning, with 10,000 absentee ballots left to count. By early Thursday, Trump led by 19,000 votes out of nearly 5 million cast in the state as a whole.
Trump has to win the states where he is still ahead, including North Carolina, plus either Arizona or Nevada to triumph and avoid becoming the first incumbent U.S. president to lose a re-election bid since fellow Republican George H.W. Bush in 1992.
The president appears to have grown more upset as his leads in some states have diminished or evaporated during the counting. On Thursday morning, he weighed in on Twitter, writing, "STOP THE COUNT!"
Although he has no authority over ballot counting, Trump later added, "ANY VOTE THAT CAME IN AFTER ELECTION DAY WILL NOT BE COUNTED!"
Some states count ballots if they were postmarked by Election Day but arrive in subsequent days.
To capture the White House, a candidate must amass at least 270 votes in the state-by-state Electoral College. Such electoral votes are based largely on a state's population. Edison Research gave Biden a 243 to 213 lead in Electoral College votes. Other news outlets said Biden had won Wisconsin, which would give him another 10 votes.
The exceedingly close election has underscored the political polarization in the United States and the deep divisions along racial, socioeconomic, religious and generational lines as well as between urban and rural areas.
The counting and court challenges set the stage for days if not weeks of uncertainty before Dec. 8, the deadline to resolve election disputes. The president is sworn into office on Jan. 20, 2021.
"The litigation looks more like an effort to allow Trump to continue rhetorically attempting to delegitimize an electoral loss," said Joshua Geltzer, executive director of Georgetown Law's Institute for Constitutional Advocacy and Protection.
Some fellow Republicans have voiced unease over Trump's claims of voting fraud.
"The problem with throwing up unsubstantiated charges is it undermines faith in democracy," Adam Kinzinger, a Republican U.S. congressman from Michigan who was re-elected on Tuesday, told CNN.
Biden, a 77-year-old former vice president and U.S. senator who has spent a half century in public life, predicted victory on Wednesday and launched a website to begin the transition to a Democratic-controlled White House.
Trump, 74, is seeking a second four years in office after a tumultuous first term in which he has been impeached but acquitted in a Senate trial, put relations with NATO allies under strain and struggled to respond to the coronavirus pandemic which has killed about 230,000 Americans.
Trump's campaign called for a recount in Wisconsin, where Biden led by roughly 21,000 votes out of 3.3 million cast, a margin slim enough to entitle him to a recount. However elections experts said a recount in Wisconsin was seen as unlikely to alter the result.
His campaign also filed lawsuits in Michigan and Pennsylvania to stop vote counting. Michigan Secretary of State Jocelyn Benson, in charge of elections, called the Trump team's lawsuit "frivolous."
Trump's campaign filed a lawsuit in Georgia to require that Chatham County, which includes the city of Savannah, separate and secure late-arriving ballots to ensure they are not counted.
It also asked the U.S. Supreme Court to allow Trump to join a pending lawsuit filed by Pennsylvania Republicans over whether the battleground state should be permitted to accept late-arriving ballots that were mailed by Election Day.
Trump's campaign said it planned to make a statement in Las Vegas later on Thursday. A source said the campaign would announce a lawsuit that would allege that thousands of people cast ballots who no longer live in the state.
Despite Trump's allegations of fraud and an unsubstantiated charge that Democrats are trying to "steal" the election, U.S. election experts say fraud in balloting is rare.
With tensions rising, about 200 of Trump's supporters, some armed with rifles and handguns, gathered outside an election office in Phoenix, Arizona, on Wednesday following unsubstantiated rumors that votes were not being counted.
In Detroit, officials blocked about 30 people, mostly Republicans, from entering a vote-counting facility amid unfounded claims that the vote count in Michigan was fraudulent.
Anti-Trump protesters in other cities demanded that vote counting continue and there were arrests in Portland, Oregon, as well as New York, Denver and Minneapolis. Over 100 events are planned across the country between Wednesday and Saturday.
By early on Thursday, Biden had drawn about 3.6 million more votes than Trump nationwide. Trump defeated Democrat Clinton in 2016 after winning crucial battleground states and securing the Electoral College win even though she won about 3 million more votes nationwide.
(Additional reporting by Steve Holland, Susan Heavey and Doina Chiacu in Washington, Mimi Dwyer in Phoenix, Tim Reid in Los Angeles, Tom Hals in Delaware and Kanishka Singh in Bengaluru Writing by Will Dunham and Andy Sullivan Editing by Scott Malone and Alistair Bell)
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