By Andy Sullivan

WASHINGTON (Reuters) - In a blow to Republican presidential candidate Donald Trump, a U.S. judge on Thursday upheld a Pennsylvania state law that could make it difficult for his supporters to monitor Election Day activity in Democratic-leaning areas.

Trump has repeatedly said Tuesday's presidential election may be rigged, without providing scant evidence, and has urged supporters to keep an eye out for signs of voting fraud in Philadelphia and other heavily Democratic areas.

Democrats worry that could encourage Trump supporters to harass minority voters in a state that could determine whether Trump or his Democratic opponent, Hillary Clinton, wins the presidency. Voting-rights advocates said they are already receiving reports of harassment.

Democrats have launched a legal blitz of their own in an attempt to shut down Trump's poll-watching efforts in Pennsylvania and three other battleground states, arguing in lawsuits that Republican monitoring efforts amount to "vigilante voter intimidation" that violates federal law. They filed a fourth lawsuit in North Carolina on Thursday.

Democrats are also trying to stop the Republican National Committee from supporting the poll-watching efforts of the Trump campaign or state parties.

Those cases have not yet been resolved.

The RNC has said in legal motions that it is not involved in poll watching, which would violate a long-standing court order. State parties have argued that they are engaged in legitimate efforts to make sure the election is conducted accurately, while Trump's vice presidential running mate, Mike Pence, and his campaign manager, Kellyanne Conway, said they misspoke when they told media outlets that the campaign was working with the RNC on poll-watching efforts.

In Pennsylvania, Trump's poll-monitoring plan faces a significant hurdle because state law requires partisan poll watchers to perform their duties in the county in which they are registered to vote.

That could make it difficult to recruit monitors in places like Philadelphia, where Democrats outnumber Republicans by a ratio of eight to one. The city has 120,000 registered Republicans and 1,685 voting locations.

The Pennsylvania Republican Party sought to suspend that requirement so that poll monitors could come from anywhere in the state, which would enable them to bring in supporters from suburban and rural areas where Trump has stronger support.

But U.S. District Judge Gerald Pappert said that would be too disruptive to change the law less than a week before Tuesday's vote.

The Republican Party of Pennsylvania did not respond to a request for comment.

Republican training materials submitted as evidence in several cases show the party is instructing poll monitors not to interact directly with voters, but to contact officials if they see a problem.

That appeared to be the message in southern Ohio as well, where Trump supporter Becky Covey said the observers she had recruited were told not to interfere with voting activity.

"People think they're going to be a watchdog, but that's not their job," Covey said.

Those guidelines could have little influence on Trump supporters who decide to engage in anti-fraud efforts of their own on Election Day.

The Oath Keepers, a paramilitary group, plans an undercover effort to monitor voting locations, while Trump ally Roger Stone is mobilizing supporters to conduct an exit poll to double-check election results. One right-wing group told the news website Politico that it has already installed hidden cameras in Philadelphia polling stations.

With early voting underway, civil rights advocates said they were already receiving reports of intimidation and harassment.

Palm Beach County, Florida, plans to station law enforcement officers at an early-voting site through Election Day after fielding complaints about bullhorn-wielding Trump supporters getting too close, according to ProPublica.

Democrats in Nevada alleged that Trump supporters have yelled at voters and tried to block them from entering early-voting sites, while civil-rights groups in North Carolina and Texas said they have received reports of intimidating behavior at early voting sites.

"We are seeing an uptick in the number of complaints compared to 2012," said Kristen Clarke, president of the Lawyers' Committee for Civil Rights Under Law, a watchdog group.

(Additional reporting by Emily Flitter in Portsmouth, Ohio; editing by Jonathan Oatis and Tom Brown)