Michigan Democrats will deliver their verdict in a bruising primary showdown that pits an establishment-backed congresswoman against a left-wing insurgent, exposing bitter divisions over the party’s future.

Moderate four-term lawmaker Haley Stevens is presenting herself as the safe bet against former public health official Abdul El-Sayed, who has been channelling grassroots frustration with Washington politics.

The outcome of Tuesday’s closely watched vote could shape the party’s direction well beyond Michigan ahead of November’s midterm elections.

The open Senate seat is almost a must-win for Democrats if they want to take control of Congress’s upper chamber and try to thwart Trump’s agenda.

The seat is currently held by a retiring Democrat and the last time a Republican was elected to this Senate seat was in 1994.

But in presidential elections it is one of the crucial swing states – Michigan voted for Democrat Joe Biden in 2020 but Trump in 2024.

Polls in the state opened at 07:00 EDT (12:00 BST) and closed at 20:00 EDT.

Whoever wins the primary will compete against Trump ally Mike Rogers in the midterms on 3 November.

Both Stevens and El-Sayed have been laying out starkly different visions for Michigan and for the country.

El-Sayed, 41, the son of Egyptian immigrants, would be the first Muslim senator in the US, if elected.

He’s been endorsed by leading left-wing politicians such as Vermont Senator Bernie Sanders and New York City congresswoman Alexandria Ocasio-Cortez.

El-Sayed has campaigned on issues such as Medicare for All, taxing billionaires, and abolishing US Immigration and Customs Enforcement (ICE).

Polling from the last year suggests healthcare costs remain among the top issues for voters this year, alongside the rising cost of petrol, groceries and housing.

El-Sayed previously told the BBC that if the Democratic Party had more concerns for everyday people “they’d be fighting on the same policies I’m fighting for”.

Despite his left-wing stances, El-Sayed is not a member of the ascendant Democratic Socialists of America (DSA) wing of the party and rejects the socialist label as unhelpful.

But he is still taking lessons from party colleagues who are democratic socialists, especially New York City Mayor Zohran Mamdani, whose social media prowess and focus on affordability helped propel him to victory last year.

But while left-wing Democrats have seen a wave of victories during this summer’s primaries – including in Colorado – it remains to be seen whether they can woo back disillusioned working-class voters who flocked to Trump in 2024.

Stevens, 43, meanwhile, has pitched herself as the more experienced and electable candidate.

She represents a more pragmatic, centrist Democratic platform that the party’s leaders hope will appeal to moderate voters.

She has earned the endorsement of Michigan Governor Gretchen Whitmer, as well as outgoing Senator Gary Peters.

Stevens has campaigned on the promise of saving manufacturing jobs in Michigan. She also says she is better suited to take on Rogers in the general election, arguing that El-Sayed’s left-wing positions would make him a weaker opponent.

In a statement to the BBC, a spokesperson for Stevens’ campaign said the congresswoman would “fight for all Michiganders from every zip code to lower costs, protect manufacturing jobs, and take on Donald Trump’s corruption and abuses of power”.

She has also gone viral for her strong Midwestern accent, which has become a major topic of discussion in the race. She addressed the internet memes at a campaign rally, calling those that dissect her voice “coastal elites”.

“Let’s be real. They’re making fun of Michigan,” she said. “They’re thinking they’re better than us.”

El-Sayed has also spoken out against US involvement in the Middle East, including aid to Israel.

Recent polling suggests a growing portion of voters think the US is too supportive of its longtime ally Israel in the wake of the Iran war.

The issue has resonance in Michigan, which is home to the second largest Arab-American population in the US.

El-Sayed has called Israel’s actions in Gaza a genocide, which Israel denies.

He has also been quick to invoke his opponent’s ties to Aipac (the American Israel Public Affairs Committee), which has spent almost $30m to back Stevens in the race.

Like many more moderate Democrats, Stevens is staunchly pro-Israel and has said she supports upholding US aid to Israel without conditions. She has also criticised Prime Minister Benjamin Netanyahu, earning a rebuke from him in a CNN interview.

While the Democratic Party has been mostly united against Trump’s war in Iran, the Israel-Gaza war continues to present a dilemma for Democrats.

They have historically depended on votes from both the Jewish and Arab communities to win elections, said Larry Sabato, the director of the University of Virginia Center for Politics.

“They have to make a choice,” he said. “Both sides are trying to make it as painful as possible.”

As polls opened on Tuesday, the justice department announced it was sending election monitors to four Michigan cities to supervise the vote.

While elections are run by US states and local governments, federal laws govern some voting practices. Political parties and candidates typically have their own election monitors at polling sites.

The move, announced by the department’s civil rights division, comes at a time when the Trump administration has sought to increase the federal government’s involvement in elections.

“Our primary monitoring efforts continue today as we seek to ensure free and fair elections nationwide,” Harmeet K Dhillon, who heads the division, said in a statement, which also urged the public to report possible violations.

Trump has repeatedly made unsubstantiated claims of widespread fraud in US elections. In March, he signed an executive order seeking to severely restrict mail-in voting and he has also pushed lawmakers to pass new legislation that would radically reform electoral voter registration procedures.

So far, lawmakers have not passed the proposed legislation and his mail-in order has been blocked in federal courts as he appeals.

Tuesday’s monitoring efforts in the cities of Detroit, Hamtramck, Lansing, and East Lansing could be a precursor to wider plans by the Trump administration for November’s midterm elections.

Additional reporting by Kwasi Gyamfi Asiedu