WASHINGTON - Joe Biden faces scrutiny over Ukraine, Elizabeth Warren is surging and Bernie Sanders had a heart attack, but recent developments in the Democratic presidential battle are getting minimal attention, swamped by Washington's impeachment obsession.

Most Democratic candidates are pounding the pavement in early voting states like Iowa, but they are finding little oxygen for the nomination race.

Broadcast and cable news have been dominated by a burgeoning Ukraine scandal that has engulfed the White House and prompted House Democrats to launch an impeachment investigation of possible abuse of power by Trump.

The drama threatens to overshadow what could be a pivotal moment in the Democratic nomination race: Tuesday's three-hour debate with the party's top 12 candidates.

The fourth showdown of the cycle will feature the Democratic dozen clashing on everything from health care and climate to jobs, gun control and foreign policy, as they seek to catch fire with voters.

Under normal circumstances the pre-debate chatter would center around whether Biden gets more aggressive against rising star Warren, if Sanders can rebound after health problems, or how second-tier candidates like Senator Kamala Harris or ex-congressman Beto O'Rourke can mount a comeback.

But talk of Trump's fate is the stark new reality, and for lesser Democratic candidates struggling for a breakout moment, their path to the nomination just got harder.

"There is a lot of this impeachment inquiry taking over the narrative, and we're going to see how these Democrats will be able to shape that to their advantage," Mitchell McKinney, director of the Political Communication Institute at University of Missouri, told AFP.

The candidate most impacted by the Ukraine crisis is Biden. A whistleblower complaint at the center of the inquiry alleged that Trump pressured Ukraine's leader on a July 25 call to investigate the US president's Democratic rival.

Trump says he acted appropriately, but he has relentlessly hammered Biden and his son as "corrupt."

McKinney said Biden could flip the attention to his advantage, convincing voters that he's the Democrat Trump fears most in 2020.

"Biden must demonstrate his aggressiveness and ability to take on the issue of Donald Trump coming at him directly about him and his son," McKinney said.

He has. Saying Trump "betrayed this nation," Biden on Wednesday called for the president's impeachment for the first time.

"Let me make something clear to President Trump: I'm not going anywhere," he added Friday on Twitter. "You're not going to destroy me."

Warren called for Trump's impeachment months ago, perhaps signaling that, while Biden earlier hedged his bets, she recognized the urgency that a growing number of Democrats feel about ousting Trump.

The 70-year-old progressive senator has surged in recent weeks, drawing virtually even with Biden in polling -- and raising the likelihood that debate rivals will go after her.

Onstage Democrats may also pounce on Biden, and argue that his son's work for a Ukrainian energy company while Biden was vice president has at least the appearance of a conflict of interest. -AFP