WASHINGTON — President Donald Trump is planning to return to Pennsylvania on Monday, May 20, for a rally, his presidential campaign announced Monday.

The event will be held in Montoursville, near Williamsport. While it is ostensibly in support of State Rep. Fred Keller, a Republican running for an open congressional seat in a special election this month, it comes as Democrats, and especially former Vice President Joe Biden, have increased their appearances in the Keystone State, one of the most critical states in the 2020 election.

Sen. Elizabeth Warren (D., Mass.) spoke to the teachers’ union in Philadelphia on Monday and Biden is due in the city for a rally Saturday, two days before Trump’s event. Biden also held a rally in Pittsburgh late last month and Sen. Bernie Sanders (Ind., Vt.) also visited the Steel City and Bethlehem in April. And voters across the state will head to the polls Tuesday for primary elections in municipal, judicial and school board races.

Since Trump won Pennsylvania in 2016 by 44,000 votes, or less than 1 percentage point, the state has been considered a “must-win” for Democrats.

“President Trump will be returning to Pennsylvania to share his long litany of achievements on behalf of the American people," said a statement from Michael Glassner, chief operating officer of Trump’s reelection campaign. “In addition to celebrating his accomplishments, President Trump looks forward to supporting State Rep. Fred Keller in his race to become the next congressman for Pennsylvania’s 12th District.”

Keller is competing with Democrat Marc Friedenberg for the seat vacated by former U.S. Rep. Tom Marino, a Republican. It is a deeply Republican district.