What to Know Pennsylvania is a crucial state in recent presidential elections. Barack Obama won the state twice, and Donald Trump won the state in 2016.

President Trump took Pennsylvania's 20 electoral votes by beating Hillary Clinton by less than 50,000 votes.

Turnout by Democrats in southeastern Pennsylvania will be key to whoever wins the nomination to challenge Trump next year.

Seven Democrats running for president, including frontrunners Joe Biden and Bernie Sanders, will be in Philadelphia on Tuesday to speak before the city's largest coalition of unions.

Biden, the former vice president, and Vermont U.S. Sen. Sanders, along with U.S. Sen. Amy Klobuchar, U.S. Rep. Tim Ryan, businessmen Tom Steyer and Andrew Yang and author Marianne Williamson will speak about their labor platforms before members of the Philadelphia Council of the AFL-CIO.

A third frontrunner in the race for the Democratic nomination, Massachusetts U.S. Sen. Elizabeth Warren, is not expected to be part of the event. A spokeswoman for the AFL-CIO said all Democratic candidates were invited, but referred questions about candidates' attendance to their campaigns.

Pennsylvania, and its 20 electoral votes, will once again play a major role in who wins the 2020 presidential race. Barack Obama won the state twice and Trump won the state in 2016. A final vote count showed Trump beat Clinton by fewer than 50,000 votes, or less than 1 percent.

The event takes place Tuesday afternoon at the Pennsylvania Convention Center in Center City.

The summit is taking place just two days after the union of autoworkers at General Motors in Michigan announced strike plans, the largest labor action by workers in the auto industry in a decade. They are striking after negotiations for a new contract with GM fell apart over wages and benefits.

The power of organized labor and the importance of the Pennsylvania electorate in presidential elections gives candidates a unique chance to connect with two constituencies at the same event, labor leaders said in advance of the AFL-CIO summit.

"It's no secret that winning Pennsylvania will be key to winning the Presidency — and that working-class voters hold that key. This summit will showcase frank talk between Democratic candidates and union members from every industry about which of them will do the best job standing up for working people in the White House," Pat Eiding, President of the Philadelphia Council, AFL-CIO, said in a statement. "On Tuesday, September 17, Pennsylvania's working families will begin to decide who, as the Democratic nominee for President, is best suited to bring about the changes necessary to deliver more union jobs and greater equality to our state and to our nation."

The local AFL-CIO is a coalition of more than 100 local labor unions in the Philadelphia region, representing nearly 200,000 workers in the public and private sectors and building and construction trades.