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WESTERVILLE, Ohio –  For the first time on Tuesday night, a record-setting 12 candidates are slated to face off on the same stage at a presidential primary showdown, topping the 11 Republican contenders who shared the spotlight four years ago at a 2016 GOP presidential primary debate.

But, that's just the first of many factors that could make this showdown – the first in a month – stand out.

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"Since the last Democratic debate, the presidential race has dramatically shifted. Tonight is the first major gathering since the opening of the impeachment inquiry," former Democratic National Committee chair and Fox News contributor Donna Brazile said.

The debate also comes as Joe Biden lost his long-held status as the unrivaled front-runner. The former vice president suddenly has been neck-and-neck with Sen. Elizabeth Warren of Massachusetts for the top spot in many of the recent national and early-voting-state polls. And, Biden was left in the dust in the race for campaign cash, with both Warren and fellow progressive champion Sen. Bernie Sanders of Vermont each topping him by roughly $10 million in the July-September third quarter of fundraising.

A look at the stage at Westerville, Ohio, hours before the fourth-round debate Tuesday.

A look at the stage at Westerville, Ohio, hours before the fourth-round debate Tuesday.

The showdown also comes two weeks after the 78-year old Sanders suffered a heart attack. The debate is set to mark his return to the campaign trail and with questions regarding his health and stamina, the independent senator making his second straight White House run will be under the microscope.

With all that in mind, here are eight things to watch at the debate.

Impeachment

The debate is the first held since House Democrats launched a formal impeachment inquiry into President Trump's July 25 call with Ukrainian President Volodymyr Zelensky, in which Trump asked Zelensky to investigate Biden and his son Hunter over their dealings in the eastern European country. Fueled by whistleblower complaints and a transcript of the call released by the White House, Democrats argued the president was pressuring a foreign country to interfere in a U.S. election.

Adding to the controversy was the fact that before the phone call, millions in U.S. military aid to Ukraine had been put on hold; it ultimately was released in September. Despite allegations that Trump was using that money as leverage, the president insisted repeatedly that he did nothing wrong. He said there was no "quid pro quo" and on numerous occasions has described his conversation with the Ukrainian leader as "perfect."

While all the candidates on the stage supported impeaching the president, the impeachment inquiry clearly will overshadow the showdown -- but will it dominate the debate to the detriment of other top issues in the Democrats' presidential nomination race?

"There is a real danger that the substance of the debate is limited by the impeachment inquiry," warned veteran political scientist Wayne Lesperance, the vice president of academic affairs at New England College.

Biden and Biden

While only one Biden will be on the stage, another certainly will be part of the conversation. While the former vice president is expected to use impeachment to frame the race as a fight between himself and the Republican incumbent, he likely will face questions about his son just hours after Hunter Biden's first public interview since the start of the Ukrainian controversy.

As the president has fought back against impeachment, he and his allies instead have tried to put the spotlight on the Bidens.

Hunter Biden served on the board of a Ukrainian energy company while his father was vice president. Trump and fellow Republicans have questioned how Biden – as vice president – pushed in 2016 for the dismissal of a Ukrainian prosecutor who was looking into corruption at the company. The prosecutor – who had faced widespread accusations of overlooking corruption in his own office – later was dismissed.

Biden, defending his actions in Ukraine, said, "there's no truth to [Trump's] charges and attacks against me and my son. Zero."

Trump's also repeatedly raised questions about Hunter Biden's role on the board of a Chinese private equity firm. This past Sunday, Hunter Biden resigned from the company – and on Tuesday morning – just hours before the debate – broke his silence in an interview with ABC News' "Good Morning America."

The younger Biden maintained that he did nothing improper while he served on the board of the Ukrainian energy company. But he acknowledged that accepting the lucrative position was, in retrospect, "poor judgment."

Some of Biden's lower-tier rivals, looking for a breakout moment, may be tempted to criticize him over the optics of having his son sit on the boards of foreign companies while he served as vice president. However, it may be a slippery slope, as none of the Democrats seeking to unseat Trump may want to play into his playbook of targeting the Bidens.

Sanders' health

"I'm ready," Sanders emphasized in an email to supporters on Monday.

While all the candidates will be under the spotlight, it will shine extra bright for Sanders, who would have a unique challenge in needing to showcase that he hasn't lost a step, and quell concerns that he may not be up to speed.

Analysts pointed out Sanders would need to show strength and agility, but the question for his rivals would be whether it's a fair game for them to raise concerns about his health. Questions were raised about the late Sen. John McCain's medical condition during the 2008 campaign and about the health of both candidates four years ago.

Sanders vs. Warren

Aside from some sniping by some of his top advisors, Sanders has not criticized Warren, as she's mostly passed him by in the polls the past two months.

That's why his labeling of his friend and populist senator – who's produced one progressive policy proposal after another – as a "capitalist" made so much news this weekend.

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"There are differences between Elizabeth and myself," Sanders said during an interview Sunday on ABC News' "This Week." "Elizabeth, I think, as you know, has said that she is a capitalist through her bones. I'm not."

Will Sanders continue to draw contrasts with the rising Warren at the debate?

A former Sanders adviser – who for months raised alarms that Warren, rather than Biden, was the real threat – said the drawing of distinctions may be a necessity.

"Unfortunately, it's a sign of concern on the part of Bernie and his camp that Warren is beating him with the voters that he needs," noted Kurt Ehrenberg, a top adviser for Sanders in New Hampshire and the 2016 and 2020 campaigns who recently parted ways with the campaign.

A more aggressive Biden?

The former vice president seemed to get a second wind last week during two speeches in New Hampshire, when Biden gave a full-throated thrashing of Trump as he called for the president's impeachment for the first time. Will that aggressive style of campaigning continue on the debate stage in Ohio?

"One of the problems I'm finding, I've got to be more aggressive," Biden acknowledged during a fundraiser late last week in California. "I've had some difficulties on knowing to counterattack."

A senior Biden campaign adviser told Fox News hours before the debate that "what you'll hear tonight from the VP is that first and foremost, we have to keep the focus on Donald Trump's unprecedented abuse of power."

Back in the spotlight

Health care – specifically the intra-party battle over "Medicare-for-all," immigration, race relations and other domestically focused issues – has dominated the discourse at the first three rounds of debates. Foreign policy mostly has been relegated to the garbage time late in the showdowns, when the number of viewers dwindled.

But, with Turkey's attack on northern Syria, unfolding soon after the president's move to withdraw U.S. troops who had been fighting alongside Kurdish forces against ISIS militants, questions about Trump's record on the world stage could be back in the spotlight.

That may benefit the candidates with foreign policy experience. Besides the obvious – Biden – such a discussion could give the two veterans in the field who've served in the wars in Iraq and Afghanistan – South Bend, Ind., Mayor Pete Buttigieg and Rep. Tulsi Gabbard of Hawaii – a moment to shine.

Looking for a breakout moment

The debate could provide the opportunity the mid-tier candidates have been seeking – a standout moment in front of millions of viewers with less than four months to go until the voting begins in Iowa. Buttigieg, Sens. Kamala Harris of California, Cory Booker of New Jersey and Amy Klobuchar of Minnesota, and former Rep. Beto O'Rourke of Texas all have been looking to gain some traction and break up the narrative that the race has been a three-candidate contest between Biden, Warren, and Sanders.

So will businessman and billionaire environment and progressive activist Tom Steyer, who's making his first appearance on the debate stage.

Buttigieg – in a likely tease of things to come – took aim at Warren and Sanders for proposing to scrap private insurance as he touted his Medicare-for-all-who-want-it plan a new digital ad hours before the debate.

"Bernie Sanders and Elizabeth Warren believe that we have to force ourselves into Medicare for all, where private insurance is abolished," the narrator says in the ad.

Unpredictable moments

Gabbard made headlines in the second round of debates in July when she took aim at Harris, firing away at the former California attorney general's record as a prosecutor. Last week, she made headlines when she threatened to boycott the debate to protest the "rigging" of the debates and the race for the nomination by the Democratic National Committee and its media partners.

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Gabbard announced Monday morning that she would take part in the debate after all, and her planned appearance – after failing to qualify for the September showdown – had analysts wondering what she had next up here sleeve.

Former Housing and Urban Development Secretary Julián Castro also has been looking to make some news. Castro – whose attacks on Biden's age and memory at the September debate backfired, according to many analysts – is set to appear on the stage for what could possibly be the last time.

With the thresholds rising for the November showdown, Castro made a fundraising pitch to supporters, telling them "I refuse to let tonight be my last time on the debate stage."

A big debate moment would give him a boost to reach the new qualifying criteria.

Paul Steinhauser is a politics reporter based in New Hampshire.