A Closer Look at the Pennsylvania Primary

— -- Pennsylvania is the biggest delegate prize out of the five Northeastern states voting tomorrow.

The small state, known for its rich history as one of the 13 original colonies, will be a battleground for the remaining presidential candidates.

Here’s a closer look at the primary in the Keystone State:

Latest Polling

Donald Trump and Hillary Clinton are ahead of their competition by double digits in the latest Pennsylvania poll.

In a NBC/Wall Street Journal/Marist poll released April 24, Trump leads his rivals with 45 percent, with Ted Cruz and John Kasich trailing at 27 and 24 percent, respectively.

On the Democratic side, the same poll shows Clinton at 55 percent and Bernie Sanders at 40 percent.

GOP Delegate Selection Process

Pennsylvania's Republican Party has an unusual method of picking delegates to send to the convention.

In total, 71 delegates are up for grabs for the Republican candidates. The candidate who garners the most votes in the primary gets 17 of the delegates, who are bound on the first ballot to vote for them at the GOP convention this summer. These 17 bound delegates are made up of three RNC members and 14 members of the state Republican Party.

Then, there are the remaining 54 delegates, which are elected directly on the GOP primary ballots handed out Tuesday in each congressional district without being bound to a candidate, meaning they are up for grabs at the convention. Voters in each of Pennsylvania’s 18 districts will pick three delegates to represent them at the GOP convention on Tuesday’s ballot.

The Democratic candidates have a much more straightforward process, with 210 delegates up for grabs, 189 of which are pledged delegates whom voters pick on the ballots. The remaining 21 are superdelegates that are picked by the party.

Closed Primary

Pennsylvania's primary is a closed primary, meaning only registered Democrats can vote in the Democratic primary and only registered Republicans can vote in the GOP primary. Independents will not be able to vote in either Republican or Democratic primaries.

Unregistered and unaffiliated voters had until March 28 to register with a party.

While Sanders has won in caucuses, which are all closed, Clinton has won every state so far that's held a closed primary.

Candidate Action/Visits

Each Republican and Democratic candidate has paid a visit to Pennsylvania this month, and all presidential candidates, with the exception of Cruz, will be in the Keystone State today.

Sanders will be campaigning in Pittsburgh today, joined for the first time by Sen. Jeff Merkley, who was Sanders’ first Senate endorsement.

Kasich will also hit the trail in Pennsylvania today, hosting a town hall in his hometown of McKees Rocks. The Ohio governor was endorsed by The Philadelphia Inquirer yesterday, adding to the list of his more than 50 newspaper endorsements.

Cruz turned his attention to Philadelphia early, hosting a watch party in Philadelphia on April 19, the night of the New York primary. But over the weekend, Cruz was back in the state and focused on attacking his main rival, Trump.

"The eyes of the entire country are on Pennsylvania right now. Pennsylvania has a platform, has a megaphone to speak to the country and we face a choice, do we want to nominate a candidate who’s a phony?” Cruz asked a boisterous crowd in Pittsburgh Saturday. “Who is telling us he is lying to us? Or do we want to get behind a strong, positive, optimistic, forward-looking conservative campaign?”

Trump, meanwhile, took some not-so subtle jabs at Clinton during his stop in Harrisburg, Pennsylvania.

"I don’t know, I think she’s qualified, I guess. But that doesn’t mean she’s good," Trump said speaking at the Farm Show Complex and Expo Center on April 21. "Bernie Sanders, not me, said she’s not qualified. So now I’m gonna say she’s not qualified. OK."

On Saturday, Clinton was in Scranton, the town where her grandfather worked at a lace mill and her father’s hometown where she spent summers as a child. She was joined on the trail Saturday by her two brothers Hugh and Tony Rodham.

Clinton plans on holding a primary night event at the Philadelphia Convention Center.

ABC News’ Ben Gittleson, Jordyn Phelps, and Liz Kreutz contributed to this report.