A strike by the staff of the nation’s largest teachers’ union has prompted President Biden to cancel a speech on Sunday in Philadelphia, where he was scheduled to address thousands of delegates to the union’s annual convention.

The staff union of the National Education Association began its strike on Friday, citing management’s revocation of holiday overtime pay for the Fourth of July holiday and its refusal to provide information on $50 million in outsourced work that may have previously been done by N.E.A. staff. The strike has shut down the last three days of the four-day convention, as delegates declined to cross a picket line.

Mr. Biden’s campaign said he would not do so, either. “President Biden is a fierce supporter of unions and he won’t cross a picket line,” a statement from his campaign said, adding that the president was still planning to travel to Pennsylvania over the weekend.

The National Education Association has about 2.5 million members nationwide, not including retirees, according to a recent government filing. The staff union says it represents more than 350 employees assigned to the union’s headquarters in Washington.

The staff union, the National Education Association Staff Organization, voted to authorize a strike in April, and its three-year contract expired in late May. It waged a one-day walkout in June. “N.E.A. has abandoned its union values with its actions at the bargaining table,” said the president of the staff union, Robin McLean, in a statement. “N.E.A. would rather cancel a multimillion-dollar convention than comply with labor law.”

The N.E.A. said in a statement that it remained “fully committed to a fair bargaining process” and accused the staff union of circulating “misinformation” that “not only misrepresents the facts but also undermines the integrity of our ongoing efforts to honor a fair bargaining process.”

The union added that the association offered generous benefits and competitive salaries, saying its current proposal would raise the average salary of staff union members to about $133,000 from about $124,000. The staff union said that salary increases had lagged for years and that most members would see an increase of less than 2 percent per year under the proposal.

Strikes by unions’ staff members are not unheard of. Employees of the United Food and Commercial Workers union held a one-day walkout in May. The staff of a large local of the Service Employees International Union in California waged a two-week strike in 2022.