WASHINGTON, Oct 30 (Reuters) - A judge in the swing state of Pennsylvania extended until Friday the deadline for Bucks County voters to request a mail-in ballot after Donald Trump's campaign sued alleging that some supporters seeking ballots were improperly turned away.

In the lawsuit filed on Wednesday, the Trump campaign along with Republican U.S. Senate candidate Dave McCormick and the Republican National Committee said a county office sent voters home on Tuesday who formed long lines ahead of a 5 p.m. deadline to apply for a mail-in ballot.

Voters in Bucks County, which narrowly voted for Democratic President Joe Biden in the 2020 election, will have until Friday to apply for, receive or return a mail-in ballot, according to an order from Judge Jeffrey Trauger.

Pennsylvania is one of seven states, known as battleground states, likely to decide Tuesday's presidential election. Its 19 electoral votes are seen as crucial for either Trump or his Democratic opponent, Vice President Kamala Harris.

"This is a landmark victory that will allow our voters to cast those votes pursuant to Pennsylvania law," RNC chair Michael Whatley told reporters, adding that he did not know if the three-day extension would benefit Democrats or Republicans.

Democrats have leaned heavily on mail-in voting in recent elections, and Trump to this day has made unfounded claims that voter fraud cost him the 2020 election and that mail-in votes are susceptible to fraud.

Bucks County, north of Philadelphia, said in a statement it was pleased to extend the deadline and urged the state legislature to adopt reforms to add clarity to election laws.

"We believe strongly in voter enfranchisement," the statement said.

The lawsuit alleged that some voters waiting in line were told by security officials to leave the facility before the 5 p.m. deadline. State officials previously said anyone in line by 5 p.m. would have an opportunity to apply for a ballot.

Al Schmidt, Pennsylvania’s top election official, said in a video posted on X on Wednesday after the lawsuit was filed that videos of the scene shared on social media “lack proper context or were inaccurate.”

Schmidt said elections officials are required to verify the identities of voters receiving ballots, a process that can be time-consuming.

Trump and his Republican allies have launched a flurry of lawsuits ahead of the election, many of which have sought to place restrictions on casting and counting of mail-in ballots

Democrats and democracy advocates have said the Republican legal campaign appears to be aimed at laying the groundwork to contest the results if Trump is defeated. The RNC has said the cases are aimed at ensuring the integrity of the vote.

The RNC asked the U.S. Supreme Court on Monday to block Pennsylvania from counting mail-in ballots rejected because of a voter mistake.

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Reporting by Andrew Goudsward; Editing by Scott Malone and Howard Goller