PHILADELPHIA (Reuters) - U.S. President Joe Biden and Republican challenger Donald Trump go before voters on Tuesday's primaries in the battleground state of Pennsylvania, one of the last tests of voter enthusiasm ahead of November's general election.

Pennsylvania, with 19 Electoral College votes toward the 270 needed to win, is a top prize and a toss-up in the 2024 presidential election featuring a Biden-Trump rematch.

In 2020, Biden won Pennsylvania by less than 1.5%, or roughly 80,000 votes. Trump beat Democrat Hillary Clinton there by fewer than 45,000 votes in 2016.

Biden clinched the Democratic nomination and Trump secured the Republican nod in early March, and neither faces serious opposition on the primary ballot. Still, the pair have visited the state in recent weeks and focused on the general election rather than Tuesday's vote. Biden was born and spent part of his childhood in the state, and has for decades been a fixture in the politics of neighboring Delaware.

Incumbent U.S. Senator Bob Casey, a Democrat, and his Republican challenger, Dave McCormick, are both expected to breeze through Tuesday's primaries. The race, which opinion polls show as close, could help decide whether Democrats retain control of the Senate.

Muslim and Arab-American voters are mounting an "Abandon Biden" campaign in Pennsylvania to protest the president's handling of the Gaza crisis. He faced similar efforts in battleground states like Arizona, Wisconsin and North Carolina, with the biggest turnout a 13% uncommitted vote in Michigan's primary.

Organizers in Pennsylvania are aiming to get 40,000 write-in "uncommitted" votes. The results are not expected to come on Tuesday night because they will have to be entered manually.

Pennsylvania is home to a sizeable Arab-American population, but also one of the largest Jewish populations, trailing only New York, California, Florida and New Jersey, according to a U.S. Jewish population report published by Brandeis University.

Although Trump's former rival Nikki Haley, a former South Carolina governor and U.N. ambassador, remains on the Pennsylvania ballot, voting is confined to registered Republicans and the primary locks out the independent voters who favored Haley during her run.

Still, any substantive result from Haley would show that some chunk of the party continues to be unhappy with the former president, who is currently on trial on 34 criminal counts in New York.

(Reporting By Jarrett Renshaw; additional reporting by Nandita Bose and James Oliphant; Editing by Trevor Hunnicutt and Jonathan Oatis)

By Jarrett Renshaw