PHILADELPHIA (WTXF/AP) - Voters from both parties will head to the polls in the Keystone State throughout the day Tuesday.
Trump and Clinton lead polls ahead of Tuesday's primary election in Pennsylvania.
Voters will decide hotly contested Democratic primary races for U.S. Senate and state attorney general.
On the Republican side of the presidential primary, billionaire developer Donald Trump has topped opinion polls heading into the election, followed by Texas Sen. Ted Cruz and Ohio Gov. John Kasich. Former Secretary of State Hillary Clinton was the leader on the Democratic side, ahead of Vermont Sen. Bernie Sanders.
Clinton planned to remain in Philadelphia on Tuesday night to await returns.
On Monday, candidates kept up a heavy presence in Pennsylvania, with Clinton, Trump, Kasich and Sanders appearing at multiple events, sometimes on opposite sides of the state.
Pollsters expect a record Republican voter turnout.
But they expect Democrats to turn out in lower numbers than they did in 2008, when 2.3 million voters, or nearly 56 percent, cast ballots in the race between Clinton and then-Illinois Sen. Barack Obama. Clinton won Pennsylvania by about 200,000 votes.
Polls opened at 7 a.m. and will close at 8 p.m.
The Republican presidential primary is something of a beauty contest, since the 54 delegates up for grabs are not promised to the statewide winner.
Rather, the delegate candidates who are elected — three in each of Pennsylvania's 18 congressional districts — can vote for whomever they want at the convention.
For the Democrats, 127 delegates are up for grabs in the primary, and they are apportioned based on the vote in each congressional district.
For U.S. Senate, four Democrats are running for the nomination to challenge Republican incumbent Pat Toomey in November.
But as Election Day approached, it came down to a race between party-endorsed Katie McGinty and former U.S. Rep. Joe Sestak, who lost to Toomey by 2 percentage points in 2010 but was spurned by the party establishment because he is seen as a maverick.
The Democratic Party recruited McGinty, with many years as a state and national environmental policy official, and poured millions of dollars into her campaign, which was benefiting from a surge of TV advertising. The fall contest could help determine control of the U.S. Senate.
In a race for state attorney general, three Democrats and two Republicans are running to succeed Democrat Kathleen Kane. Facing trial over the alleged unlawful leaking of grand jury information, Kane decided not to seek a second term.
In congressional races, Democratic U.S. Rep. Chaka Fattah of Philadelphia is running for a 12th term in a four-way primary as he faces trial on federal racketeering and bribery charges in May.
Also, Democrats and Republicans are running to succeed retiring Republican U.S. Reps. Mike Fitzpatrick and Joe Pitts in southeastern Pennsylvania.