Former Vice President Joe Biden will join U.S. Sen. Bernie Sanders and five other Democratic candidates for president in a Philadelphia labor forum Tuesday afternoon.

Biden, the Democratic front-runner, had been slow to commit to the event at the Pennsylvania Convention Center, a perceived snub that has caused friction, especially because his campaign headquarters is four blocks from the venue.

Biden’s campaign on Monday told The Inquirer that he would appear.

It has been more than four months since Pat Eiding, president of the AFL-CIO in Philadelphia, called a meeting to tell local labor union leaders to “keep their powder dry” in presidential politics.

Eiding said in early May that a sudden groundswell of support for Biden was happening too fast. He expressed concerns then that Democratic candidates were courting big-dollar donors, but not paying enough attention to rank-and-file union concerns.

Tuesday’s “Workers’ Presidential Summit” grew from that meeting.

Sanders had been, until Monday, the only top-tier candidate on the bill Tuesday. He is positioned in second place in an average of national polling maintained by the website Real Clear Politics.

Two other candidates who qualified for last Thursday’s Democratic debate, U.S. Sen. Amy Klobuchar of Minnesota and entrepreneur Andrew Yang, are also scheduled to attend, along with billionaire activist Tom Steyer, U.S. Rep. Tim Ryan of Ohio, and New York City Mayor Bill de Blasio.

Former U.S. Rep. Joe Sestak of Delaware County had committed to attend, but then canceled so he could campaign in Iowa on Tuesday. Author Marianne Williamson also dropped out of the event.

The candidates will each appear for 20 to 30 minutes in front of a crowd anticipated to number about 2,000, with a moderator asking questions submitted through social media and the website for the event.

Union members can register for the event at workerspresidentialsummit.com, where they will be asked to identify what labor union and local they work for. Local unions have also received tickets, which they are distributing to their members.

Eiding said the event is still open to any of the 13 other Democratic candidates who have not committed to attend. The forum opens at 3 p.m. and is expected to draw union members from Philadelphia, the four surrounding suburban counties, and Southern New Jersey.

“We have this set up so that if [other candidates] call up and say they’re coming, we’ll accommodate them,” Eiding said. “We’ll make it happen.”

Biden was the only Democratic candidate who asked for a lone appearance, apart from the other candidates, Eiding said.

“I made it clear we couldn’t do it,” he said. “It would look like an endorsement.”