The three-hour debate at Ohio's Otterbein University features Joe Biden.

Twelve Democrats square off Tuesday in a crowded presidential debate overshadowed by an impeachment inquiry into Donald Trump, who has dragged chief rival Joe Biden into the Ukraine crisis.

Will the frontrunners -- Biden, Senator Elizabeth Warren and Senator Bernie Sanders -- extend their leads over the rest of the field, or will second-tier candidates desperate for a breakout moment vigorously challenge those at center stage?

It is an extraordinary moment. Washington's impeachment brawl has dominated US politics for weeks and denied Democratic candidates the attention they crave ahead of what could be a pivotal showdown in the race to see who will face Trump in 2020.

Trump's July 25 phone call to his Ukrainian counterpart seeking help in investigating the Bidens, and the discredited charge by Trump that the former US vice president intervened in Ukraine to protect his son Hunter, has roiled the race and put Biden in the spotlight.

The three-hour debate at Ohio's Otterbein University features Biden, who is struggling to maintain frontrunner status, at center stage with progressive Warren, the only candidate steadily rising in the field and who is now nipping at his heels. Both are likely to face attacks from rivals.

Next is liberal Sanders, who is under immense pressure to project fortitude two weeks after he was sidelined by a mild heart attack.

Joining them are South Bend, Indiana Mayor Pete Buttigieg; senators Kamala Harris, Cory Booker and Amy Klobuchar; ex-congressman Beto O'Rourke; entrepreneur Andrew Yang and Obama-era cabinet member Julian Castro.

Two others who missed the September debate but qualified for Ohio are congresswoman Tulsi Gabbard and billionaire activist Tom Steyer, who for two years has sought Trump's impeachment.

Guns blazing

The scandal over Trump's communications with Ukraine has engulfed the White House, and will surely color the debate proceedings.

An embattled Trump has come out with guns blazing against Biden and his son Hunter, repeatedly claiming without evidence that they are corrupt.

Biden has begun criticizing the president more forcefully, tweeting Tuesday that "Trump is leading the most corrupt administration in modern American history."

But Biden stands to lose perhaps the most in the Democratic dozen, as voters determine whether Trump unfairly targeted him and his son, and if Hunter's international work had the appearance of a conflict of interest.

Early Tuesday, Hunter Biden, who worked for a Ukrainian energy company when his father was vice president, broke his silence, telling ABC News that while he may have exercised poor judgment in his business dealings in Ukraine and China, he broke no laws.

"Did I make a mistake based on some ethical lapse? Absolutely not," Biden, 49, insisted.

Capitalist, socialist

Beyond the Trump impeachment fight, likely debate topics include health care, climate change, immigration, gun control and the economy.

Foreign policy is expected to loom larger than in previous debates, following Trump's controversial troop pullout from northern Syria, which critics including virtually all Democratic candidates say green-lighted a Turkish invasion.

Warren, meanwhile, takes the stage as the candidate on the rise, and the 70-year-old's performance will be closely watched.

Sanders, a self-declared Democratic socialist, publicly distanced himself from his friend and fellow progressive for the first time Sunday, saying there were "differences" between his outlook and Warren's.

"Elizabeth I think, as you know, has said that she is a capitalist through her bones. I'm not," he told ABC's "This Week."

Sanders, 78, is grappling with his own setback following a heart attack that put his health and age into question.

It tangentially raises concerns about Biden, who turns 77 next month and has been criticized for lacking vitality in debates.

Ohio voted twice for Barack Obama and then flipped to Trump in 2016. Democrats are aiming to take it back next year.