By Joseph Ax

NEW YORK (Reuters) - Philadelphia public transit officials may ask a federal judge as soon as Friday to force striking workers to return to their jobs on Election Day if a deal is not in place by then, an agency spokesman said on Thursday.

The work stoppage has raised fears that some voters in and around Philadelphia, which is heavily Democratic, might face difficulties getting to the polls for Tuesday's U.S. presidential election.

The agency was considering a request for an injunction to ensure voters can cast ballots in the country's fifth largest city by population, said Andrew Busch, a spokesman for the Southeastern Pennsylvania Transportation Authority (SEPTA).

About 4,700 members of Transport Workers Union Local 234 went on strike on Monday night after failing to reach a deal with SEPTA on issues including healthcare and pension benefits, shuttering bus, trolley and rail services that provide 900,000 rides a day.

Pennsylvania is a key swing state in the presidential campaign between Democrat Hillary Clinton and Republican Donald Trump.

In addition, the U.S. Senate race pitting incumbent Republican Pat Toomey against Democrat Katie McGinty is considered one of half a dozen close contests that will determine which party controls the Senate, which is currently dominated by the Republicans. Polls show McGinty with a slight lead.

The union has endorsed Clinton, but members voted last month not to suspend the strike for any reason unless an agreement is reached, said union spokesman Jamie Horwitz.

Philadelphia officials said previous strikes during elections had no effect on turnout. Every city resident lives within five blocks of his or her polling place, said Kevin Kelly, the acting supervisor of elections in Philadelphia.

But suburban voters could also be affected. While the regional rail system is running, commuters have experienced delays, and those who work in Philadelphia may have to leave early and return late, making it harder to vote.

The four counties served by SEPTA's regional rail are either Democratic-leaning or split evenly between the parties. Political analysts say the suburban vote could determine whether McGinty can eke out a victory.

"It's a pretty serious issue, and it's one that no one was counting on here," said Diane Ellis-Marseglia, a commissioner in Bucks County, a suburban area just northeast of Philadelphia that is a prime target for both Democrats and Republicans.

Stephanie Formas, a spokeswoman for Clinton's Pennsylvania operation, said in an email, "We are hopeful that both sides can come to an agreement in short order."

In an Oct. 31 strategy memo, the Clinton campaign said it would open more than 300 get-out-the-vote offices for Election Day and knock on millions of doors to ensure voters go to the polls.

Pat Poprik, chairwoman of the Bucks County Republicans, said it was possible some residents might have trouble getting to the polls between 7 a.m. and 8 p.m. due to transit delays.

"We're hoping they're able to make arrangements," she said, adding that the party would provide rides if needed.

(Reporting by Joseph Ax; Editing by Jonathan Oatis)