Tales of moderate Democrats seeking a new candidate to jump into the race to be the party's 2020 U.S. presidential candidate have swirled over the past fortnight.

Establishment Democrats are said to be nervous that a runner capable of beating Donald Trump in a year's time has yet to emerge from the party's crowded field.

Elizabeth Warren and Bernie Sanders excite the base but are seen as too liberal to win a general election, while polling front-runner Joe Biden's struggle to attract small donors is a sign of low enthusiasm for the former vice president.

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A guide to the 2020 U.S. elections

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Some reports suggested Hillary Clinton was mulling a comeback, others that Michael Bloomberg or John Kerry might make a late bid. Oprah Winfrey has "repeatedly begged" Disney boss Bob Iger to join the field, according to the Washington Post, while Sherrod Brown is another name that has been thrown into the mix. 

Early buzz around Beto O'Rourke – who quit the race on Friday – and Kamala Harris – who has been forced to cut staffing – faded quickly, while Amy Klobuchar has struggled to translate strong debate performances into polling numbers. 

And candidates such as Michael Bennet and Steve Bullock are still running but can't hit the fundraising or polling thresholds to make the debate stage.

For the moderate wing seeking a center-left candidate with the potential to inspire, that leaves Pete Buttigieg as the most promising bet with less than 100 days until the Iowa caucuses.

The 37-year-old, mayor of South Bend, Indiana, has been a fundraising hit and a frequent media presence, but until recently failed to translate money and exposure into polling numbers.

After bumping along at around five percent in the RealClearPolitics national polling averages since July, Buttigieg climbed to over seven percent at the end of October – a small rise, but when combined with a surge in Iowa is a sign that he is gaining some traction.

In Iowa, the crucial first voting state in the process to choose a nominee, Buttigieg has pulled ahead of Biden and Sanders into second place at 17 percent according to the RCP tracker. Warren leads at 22 percent, but the mayor has the momentum and is clearly confident.

"There is amazing energy behind our campaign right now," the Afghanistan veteran told ABC's This Week on Sunday.

"We're seeing it on the ground here in Iowa, we're seeing it pick up in a lot of places. And I think voters are really narrowing down their choices instead of just getting to know us.”

In an interview with Showtime's The Circus on Sunday, Buttigieg argued that the contest for the Democratic nomination will ultimately be between himself and Warren, the Massachusetts senator he has taken on over healthcare.

"I think this is getting to be a two-way. It's early to say, I'm not saying that it is a two-way. A world where we're getting somewhere is where it's coming down to the two of us."

Healthcare is set to be a dividing line in both the primaries and the general, and Warren has released a funding plan for her Medicare-for-all program since Buttigieg's jibe that the senator "has a plan for everything – except this" in the fourth Democratic debate.

"What is just not true is that hers is the only solution," Buttigieg, who has a "Medicare for all who want it" plan, told ABC. 

The network released new polling results in conjunction with the Washington Post on Sunday that suggested that while Biden, Warren and Sanders remain the clear top three, there is plenty of scope for change. 

Fifty-three percent of Democratic-leaning voters said they'd consider another candidate or didn't have a favorite.