Washington, DC – On Tuesday, a progressive Democrat will fight to save her seat in the United States Congress — against a threat from within her own party.

That Democrat, Cori Bush, faces a punishing primary challenge from county prosecutor Wesley Bell, as they both compete to represent Missouri in the House of Representatives.

But experts say their battle boils down to one central issue: how to approach Israel’s war in Gaza.

Bush, a member of the progressive “squad” in Congress, has been vocal in her opposition to Israel’s military offensive, which has claimed more than 39,600 Palestinian lives.

She maintains that the primary challenge she faces is part of a larger effort to silence critics of Israel, a longtime US ally — and sow division among Democrats.

“This is only the beginning,” Bush told the Associated Press in an interview published last week. “Because if they can unseat me, then they’re going to continue to come after more Democrats.”

Political observers have also noted that Bush’s rival, Bell, has the backing of powerful pro-Israel lobbies.

Their race is the latest Democratic primary to see a massive injection of cash from the American Israel Public Affairs Committee (AIPAC) and its affiliated super PAC, the United Democratic Project (UDP). Those two organisations have poured $8.4m into backing Bell’s bid for Congress.

In this explainer, Al Jazeera breaks down the issues at stake in the Missouri race — and what the outcome could mean for the future of the Democratic Party.

When and where is the primary?

The Democratic primary is taking place on Tuesday, August 6.

It will determine which Democratic candidate will proceed to the general election on November 5, for a chance to represent Missouri’s First Congressional District.

That district includes the city of St Louis, a major metropolitan area on the Mississippi River, near the border with Illinois. It is considered a solidly Democratic area, though Missouri as a whole tilts to the right.

Who is incumbent Cori Bush?

Bush, a 48-year-old nurse and pastor, rose to fame as an activist following the police killing of Michael Brown, a Black teenager, in the city of Ferguson, Missouri, in 2014.

She credits the horror of Brown’s death — and the protests that erupted afterwards — with propelling her to enter politics. Initially, she made two failed bids for Congress, losing a Senate race in 2016 and a race for the House in 2018.

But in 2020, her fortunes changed. She unseated 20-year veteran Lacy Clay in the Democratic primary that year and ultimately won the race to represent Missouri’s First Congressional District.

Her election was part of a wave of progressive victories across the country, including Jamaal Bowman’s upset win in New York. Other prominent progressives, including Alexandria Ocasio-Cortez and Rashida Tlaib, successfully defended their seats that year too.

Bush became the first Black woman to represent her district in Congress.

Who is her Democratic rival, Wesley Bell?

Wesley Bell has been the prosecuting attorney for St Louis County since 2018. He is the first Black man to hold the office.

Like Bush, Bell also was active during the Ferguson protests, working “directly to calm tensions between residents and the police”, according to his campaign website.

He worked on a council that liaised with the US Department of Justice to create a plan to reform the criminal justice system in Ferguson. But like his predecessor in the prosecutor’s office, Bell faced criticism for declining to bring charges against the officer involved in Brown’s killing.

Like Bush, Bell has styled himself as a progressive: Many of their policy positions broadly align. Bell, however, has criticised Bush for voting against President Joe Biden’s bipartisan infrastructure package in 2021.

What are the candidates’ positions on Israel?

One of the biggest points of divergence between Bush and Bell is their stance on Israel.

Bush was among the earliest representatives in Congress to call for a ceasefire in Gaza, and she has repeatedly supported legislation to increase pressure on Israel to stop the war.

She has also compared Israel’s military campaign in Gaza to “ethnic cleansing” and the “collective punishment against Palestinians”.

Bell, by contrast, has said the US must continue to support its ally Israel, while working towards a “peaceful resolution”.

He recently told the Associated Press that Bush’s criticism of the war was “wrong and offensive”.

“We don’t want to see any innocent Palestinians, any innocent Israelis, harmed. We want to keep the door open to a two-state solution,” he said.

Why did Wesley decide to confront Bush?

The war in Gaza has loomed large over the primary race since its beginning.

Shortly after the war began on October 7, Bush joined other progressive members of the House in calling for “an immediate de-escalation and ceasefire in Israel and occupied Palestine”.

But that stance prompted a backlash against Bush, with some of her fellow Democrats attacking her for not supporting Israel’s right to “self-defence”.

By October 30, Bell had announced he would abandon his Senate campaign to take on Bush for her House seat.

What has AIPAC’s role been?

Since Bell’s entry into the race, AIPAC’s super PAC, the United Democratic Project, has spent more than $8.4m to unseat Bush.

That accounts for more than half of the money spent outside of the campaigns’ own coffers.

The Supreme Court’s 2010 Citizens United ruling allows super PACs to spend unlimited amounts of money on political races, as long as the groups do not directly coordinate with the candidates.

In the case of the Democratic primary in Missouri’s First Congressional District, the result has been a deluge of advertisements attacking Bush or supporting Bell.

While many of the advertisements are funded by the pro-Israel super PAC, they often contain scant mention of either candidate’s position on Israel.

Outside spending has also poured in for Bush, including $2.2m from the Justice Democrats, a national progressive group. Still, overall outside spending to support Bush has accounted for only about a third of what has been spent in support of Bell.

What does it all mean?

Critics have long decried the role of unlimited outside spending in US politics, saying it allows for certain interest groups to exert outsized influence over campaigns.

For instance, a recent campaign finance analysis by Politico found that, while AIPAC receives donations from both Republicans and Democrats, it represents the “biggest source of Republican money flowing into competitive Democratic primaries this year”.

Bush’s supporters fear the spending sends a chilling message: that any criticism of Israel comes at high risk to lawmakers in the US.

The campaign to unseat Bush comes shortly after the United Democratic Project spent a historic $14.5m to successfully unseat a fellow “squad” member, Congressman Jamaal Bowman, earlier this year.

Critics also warn that the spending could produce results that are out of step with the Democratic base. Public opinion polls show broad disapproval in the US towards Israel’s military actions in Gaza as well as strong support for a ceasefire, particularly among Democrats.

“This gets to the point where it is very troubling, where outside spending can even top what the candidates spend. And so it means the candidates aren’t in charge of the campaigns,” Craig Holman, a government affairs lobbyist for the consumer rights advocacy group Public Citizen, told Al Jazeera in June.

“We’ve seen that happen occasionally before, but now we’re seeing it happen more regularly, and that is problematic.”