Thousands of Philadelphia transit workers went on strike early Tuesday, as SEPTA and the workers' union failed to agree on a new contract.

The work stoppage will idle bus, trolley, and subway service in the city for tens of thousands of commuters, and if it is prolonged could hurt voter turnout on Election Day next Tuesday.

Negotiations between SEPTA and the union representing 5,000 transit workers had continued into the night Monday as the potentially crippling strike loomed.

Authority lawyers and members of Transportation Workers Union Local 234 raced against the clock to resolve their contract dispute.

Their contract expired at 12:01 a.m. Tuesday. Absent an agreement, the TWU said that there would be no extensions and that its members would go on strike.

"We are not able to come to an agreement with SEPTA, so as of 12:01 we're on strike," TWU president Willie Brown said.

He cited pensions, health care, and driver fatigue as issues. "We need to have something to make sure our drivers aren't going down the street half asleep," he said.

He said that talks would continue and that the union was willing to go to binding arbitration.

A SEPTA spokesman said in a statement that Brown had "walked away from a contract offer that would have provided his members pay raises, enhanced pension benefits, maintained health-care coverage levels, and continued job security, while also remaining fair and affordable for the taxpayers and riders who fund SEPTA."

"We are hopeful that a tentative agreement will be reached before Election Day," the statement said. "If we foresee an agreement will not come to pass, SEPTA intends to seek to enjoin the strike for Nov. 8 to ensure that the strike does not prevent any voters from getting to the polls and exercising their right to vote."

With just hours to go, pensions remained a key point of contention, a source with knowledge of the negotiations said.

The union workers are unhappy that their pensions do not grow commensurate with pay once a worker makes more than $50,000. Managers' pensions are uncapped, and members of Local 234 also want uncapped pensions, the source said.

One of the city's Democratic heavy hitters, U.S. Rep. Robert Brady, entered the fray with just hours to go before a potential work stoppage.

He said he decided to "pop in" after hearing from a representative of Local 234 and SEPTA board members.

Updates on the situation will be posted online at Philly.com.

"It's going to hurt," Brady said of a potential prolonged strike's effect on voting next Tuesday. He said Hillary Clinton's campaign had expressed concern through Corey Dukes, director of the campaign's Pennsylvania operation.

At 11 p.m. SEPTA representatives said talks were "moving in the right direction." A union representative was less sanguine. He said TWU negotiators had still not heard what they needed to hear about scheduling issues. They were adamant that SEPTA grant workers more down time between shifts and longer breaks between routes. Drivers' breaks, about five minutes long now, barely give them enough time to use a restroom during work, union representatives have said.

The transit authority suggested that striking operators could choose to continue working and finish their shifts into the predawn hours Tuesday, but could not promise that they would. Brown said buses would finish their routes.

SEPTA has said a strike would not affect bus or trolley service outside the city, but suburban bus services would not travel into Philadelphia. Workers who depend on the Route 125 bus to get from Center City to King of Prussia, for example, will have to get to Villanova to pick up their bus.

The only alternative transit riders will have in the city is Regional Rail, which has 52 stops in Philadelphia.

Riders traveling within the city on Regional Rail will be able to board with transit fare cards at transit prices, but SEPTA warned that Regional Rail might not be an appealing alternative.

"Regional Rail is already operating at or near capacity, so we need to stress this is a very limited option," said Carla Showell-Lee, SEPTA spokeswoman.

Mayor Kenney was maintaining contact with both sides in the negotiations, said spokesman Mike Dunn, but will only intervene directly in the talks if both sides ask him to.

Philadelphia courts will run a shuttle service to transport jurors to the Municipal Services Building from 6 to 9 a.m., and then carry jurors leaving court starting at 3 p.m.

Uber is expanding access to its UberPool service, which allows riders to share an UberX with strangers for a reduced rate, into the suburbs. UberPool has been available in Philadelphia, Wilmington, and Newark, Del., and in the event of a strike will become available to riders in the surrounding counties.

On Sunday night, John Taylor (R., Phila.), chairman of the state House of Representatives' Transportation Committee, implored union representatives and SEPTA negotiators to talk all night and into Monday, if necessary, to avoid a strike.

"This would be a terrible thing for the city of Philadelphia at any time but particularly as we move toward Election Day," he said.

If there is a strike, Tuesday morning could see transit riders scrambling to get to work. City transit provides 800,000 to 850,000 trips each weekday.

Together, the Market-Frankford and Broad Street Lines move more than 311,000 people daily, and trolleys carry about 83,000. While the region's suburban bus lines will continue to operate, essentially any route with a number lower than 90 will not. SEPTA's website, septa.org, has a list of services that would be affected if there is a strike.

Amna Khalafalla, 18, who lives in Northeast Philadelphia, needs SEPTA to attend Philadelphia High School for Girls in Logan.

"I will be staying home if SEPTA does strike," she said. "My parents will go to work, so SEPTA is my only means of transportation. It will disrupt me, schoolwise. I usually get home at 8 p.m. on Tuesday because of all of my school activities, so the strike will cancel every activity for me."

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Staff writer Robert Moran contributed to this article.