For the third time in less than eight months, a special election will decide control of the narrowly divided Pennsylvania House of Representatives and provide political reinforcements to either the commonwealth’s Democratic governor or its Republican-controlled Senate.

On Tuesday, voters in the heavily Democratic 21st legislative district will choose a replacement for former state Rep. Sara Innamorato, who stepped down in July to focus on her bid to be Allegheny County’s next county executive. Her resignation bumped Democrats from a one-vote majority in the chamber to a 101-101 tie with Republicans.

The Democratic nominee is Lindsay Powell, director of Workforce Strategies for InnovatePGH, an economic development nonprofit, and a former aide to U.S. Senate Majority Leader Chuck Schumer, U.S. House Minority Leader Hakeem Jeffries, and former Pittsburgh Mayor William Peduto. She faces GOP nominee Erin Connolly Autenreith, a real estate agent and chairwoman of the Shaler Township Republican Committee. Her father, Thomas Connolly, served as mayor of nearby McKees Rocks in the 1980s.

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The winner will complete the remainder of Innamorato’s two-year term and be up for reelection in November 2024.

District 21 is located in the heart of Allegheny County in southwestern Pennsylvania and includes parts of Pittsburgh as well as the suburbs of Etna, Millvale, Reserve and Shaler to the north. Innamorato won the district in 2022 with 63% of the vote. Allegheny County has Pennsylvania's second-largest population and votes reliably Democratic, supporting Hillary Clinton and Joe Biden in the last two presidential elections with 57% and 60% of the vote, respectively. U.S. Sen. John Fetterman received 63% of the county vote in 2022 over Republican nominee Dr. Mehmet Oz.

In the 2022 midterm election, Democrats won a majority in the Pennsylvania House for the first time since 2010, but Republicans occupied more seats by the time the term began in January because of three vacancies that were created after the election. Two Democratic members resigned to assume other offices –- lieutenant governor and U.S. representative — while a third died before Election Day and was reelected posthumously.

Democrats regained their numerical majority in February after winning special elections to fill the three vacancies. Two additional vacancies, one by a Republican who was elected to the state Senate and another by a Democrat who resigned amid sexual harassment allegations, forced another round of special elections in May that would once again determine control of the House. Those elections resulted in the 102-101 Democratic edge that stood until Innamorato’s resignation in July.

Yet another special election that could determine control of the Pennsylvania House may be in the works early next year if Democratic state Rep. John Galloway is elected to a district judgeship in November, as expected.

The House is scheduled to reconvene on Sept. 26.

Here’s a look at what to expect on election night:

ELECTION DAY

The special election for Pennsylvania state House District 21 will be held on Tuesday. Polls close at 8 p.m. ET.

WHO GETS TO VOTE

Voters must be registered in House District 21 to participate in the special election. The deadline to register was Sept. 5.

DECISION NOTES

Under its current boundaries, District 21 heavily favors Democrats. Innamorato won the 2022 general election with 63% of the vote. She performed best in the southern half of the district, which includes parts of Pittsburgh, where she dominated most of the city’s 6th, 9th, and 10th wards with between 80% and 89% of the vote.

The Republican that year, Frank Perman, carried only 16 of the district’s 79 wards, all of them in Shaler Township. This year, Autenreith would have to outperform the 50%-59% Perman scored in the eastern and western parts of Shaler, as well as cut into the Democratic lead in the rest of the township, which is conceivable considering she is the local Republican committee chairwoman. But to win, she would also have to force Powell to underperform in Pittsburgh and neighboring wards, which is a tall order considering the area’s voting history.

The AP does not make projections and will declare a winner only when it’s determined there is no scenario that would allow the trailing candidate to close the gap. If a race has not been called, the AP will continue to cover any newsworthy developments, such as candidate concessions or declarations of victory. In doing so, the AP will make clear that it has not yet declared a winner and explain why.

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Pennsylvania has automatic recounts in statewide races if the margin between the top two candidates is 0.5 percentage points or less. In district races, Pennsylvania law allows recounts if three voters in the district request and pay for the recount, regardless of the winning margin.

The AP may declare a winner in a race that is eligible for a recount if it can determine the lead is too large for a recount or legal challenge to change the outcome.

WHAT DO TURNOUT AND ADVANCE VOTE LOOK LIKE

As of Monday, there were 47,682 voters registered in Pennsylvania’s House District 21, according to the Pennsylvania Department of State’s website. Of those, 59% are Democrats, 26% are Republicans and 11% are not affiliated with any party.

The AP's preliminary turnout estimate as of Thursday is 16,000 votes, based on the results of previous contests in the district as well as those of other Pennsylvania House special elections this year in comparison to the turnout in those districts in the 2022 general election.

In the 2022 general election, 27% of ballots were cast before Election Day. The Democratic incumbent won 84% of those advance votes. As of Thursday morning, 3,600 ballots had been cast, with 82% by registered Democrats and 12% by registered Republicans.

HOW LONG DOES VOTE-COUNTING USUALLY TAKE?

In the 2022 general election in District 21, the AP first reported results at 8:39 p.m. ET, or 39 minutes after polls closed. The election night tabulation ended at 10:33 p.m. ET with about 98% of total votes counted.