Voters are preparing to head to the polls for a primary election in Pennsylvania, an eastern US state that has served as a crucial battleground in recent national elections.

Republican and Democratic candidates are facing off to represent their respective parties in the upcoming midterm elections in November, a test for United States President Donald Trump’s second term that could determine control of the US Congress for the remainder of his time in office.

When is the primary? What will the voting process entail? Which races are being watched? We answer those questions and more in this brief explainer.

When is the primary?

Election Day is scheduled for May 19.

According to state election authorities, the last day to register to vote was May 4, and the last day to request a mail-in or absentee ballot was May 12.

What do people need to vote?

The state has a closed primary system, meaning voters can cast ballots only in the primaries of their registered party.

Pennsylvania does not generally require voters to present identification. However, those voting at a polling location for the first time must present a valid form of ID, such as a driver’s licence or passport.

What time do polls open?

Polls will open at 7am local time (11:00 GMT) and close at 8pm (midnight GMT).

What positions are up for grabs, and why does this election matter?

A number of races are being watched for their significance to national politics, including a gubernatorial race that could serve as a showcase for the political ambitions of incumbent Democratic Governor Josh Shapiro.

The state Democratic Party is also hoping to secure majorities in both houses of the state legislature for the first time in several decades, with the entire state House and half of the state Senate seats up for grabs in November. While Democrats currently hold a one-seat majority in the state House, they have not held a majority in the state Senate for more than 30 years.

A tracker for state primaries from the Pennsylvania news outlet Spotlight PA says that there are 228 legislative primaries this year, many of them uncontested. The tracker has placed the 16th and 36th districts as primaries that will have a competitive general election in November for control of the state senate.

Pennsylvania is also likely to play an important role at the national level in the midterm elections, with voters set to decide on candidates for all of the state’s 17 seats in the US House of Representatives. All but one of the state’s US House legislators are seeking re-election.

Voters will also choose their party’s candidate for lieutenant governor.

Who are the candidates for governor?

Both the Republican and Democratic candidates for governor are running uncontested in their primary races.

Democratic Governor Josh Shapiro, often discussed as a potential future presidential candidate, is running as an incumbent, while State Treasurer Stacy Garrity is running unchallenged for the Republican ticket.

“We all know that he’s more interested in Pennsylvania Avenue than helping Pennsylvania families,” Garrity said of Shapiro’s possible White House aspirations in a recent interview. “He thinks if he can hand Pennsylvania on a platter to the Democratic Party, then maybe they’ll take a harder look at him.”

Supporters of former Republican gubernatorial candidate and State Senator Doug Mastriano have launched a write-in campaign on his behalf against Garrity, who Trump endorsed in January.

The Trump administration announced last week that it was nominating Mastriano to serve as ambassador to Slovakia.

What about lieutenant governor?

Incumbent Austin Davis will be the Democratic candidate, while Republicans are choosing between Jason Richey and John Ventre.

Richey, an Allegheny County attorney, is Garrity’s preferred running mate, but faces a challenge from Ventre, a retired UPS executive whom Garrity’s campaign has accused of spreading false claims and misleading voters on social media, according to reporting from local National Public Radio (NPR) affiliate WESA.

Ventre shared a graphic on social media stating that he and Garrity are the Republican ticket “effective May 20”, prompting a statement from the gubernatorial candidate stating that the two have never met and slamming him for sharing a “misleading graphic”.

What other races are being watched?

The upcoming retirement of Democratic Representative Dwight Evans has set off a competitive primary for the state’s third congressional district, which represents parts of Philadelphia, the state’s largest city.

In that race, progressive candidate Chris Rabb is facing off against Ala Stanford and Sharif Street, both seen as closer to the party’s traditional wing. Rabb has been endorsed by progressive figures, including Representative Alexandria Ocasio-Cortez.

The news outlet Axios recently reported that Shapiro has used his influence to encourage support for candidates other than Rabb, who has previously criticised the governor from the left on issues such as immigration and whose pro-Palestine politics are at odds with those of Shapiro, an outspoken supporter of Israel.

What are the polls saying?

Most polls show Shapiro leading his rivals in the gubernatorial race by a sizable margin, with a February Quinnipiac poll showing him winning 55 percent of the vote against 37 percent for Garrity.

An April survey from Susquehanna Polling also shows Shapiro winning with 58 percent versus 36 percent for Garrity.