This week the bid to take on Donald Trump, the presumed Republican nominee in the 2020 presidential election, stepped up a gear.
Senator Kamala Harris officially launched her campaign in Oakland, California, gaining primetime TV coverage on an otherwise quiet Sunday afternoon. On Friday, Senator Cory Booker of New Jersey announced his bid.
But it was another well-known Democrat who ruffled most feathers. Howard Schultz, the former chief executive of Starbucks, announced he was mulling a run. Though he describes himself as a "lifelong Democrat", the billionaire businessman announced on CBS's 60 Minutes that he would run as an Independent if he decides to contest.
“We look at both parties, we see extremes on both sides,” he said. “The American people are exhausted. Their trust has been broken. And they are looking for a better choice.”
The reaction from Democrats was swift and unequivocal. "We've seen this movie before," snapped Debbie Wasserman Schultz, the Florida congresswoman who chaired the Democratic National Committee during the last presidential primary campaign. "He doesn't quite grasp the depth of the impact that that decision would have on our ability to defeat Donald Trump," she said. "It would be incredibly damaging."
Ms Wasserman Schultz was referring to past presidential runs by Independents. The ghosts of Ross Perot and Ralph Nader still stalk the collective Democratic and Republican memory. In the 1992 presidential election, Mr Perot staged a strong third-party bid, winning 19 per cent of the popular vote, which many Republicans still believe cost George HW Bush the election.
Meanwhile, Democrats are still wounded by the 2000 presidential campaign when Green Party candidate Nader received more than 97,000 votes in Florida. Given that the state was ultimately won by George W Bush by only 537 votes following one of the most contentious recounts in history, many believe Mr Nader cost Al Gore the presidency.
There are also examples from more recent history. Some Democrats will never forgive Jill Stein, who ran for the Green Party in 2016. She won more votes in Wisconsin than Mr Trump's winning margin over Hillary Clinton in the state.
Mr Schultz has hit back. He believes he represents a centrist vision that many Americans can relate to. Describing himself as a fiscal conservative but with socially-liberal views on issues such as abortion and gun control, the 65-year-old has cited the fact that 40 per cent of Americans identify themselves as Independent.
But analysts have pointed out that despite this, most voters have a favourable view of either one of the two main parties and are unlikely to vote for an Independent in anything near the numbers that could deliver victory in the electoral college system. While many Democrats believe the two-party system is flawed, they also believe that now is not the time to fix it, when the prospect of a second Trump term looms.
As well as the possibility of a Schultz bid fracturing the vote in the presidential election, the billionaire's intervention has given Democrats another headache by exposing the divisions between the more progressive and centrist wings of the party. As it faces a crowded field of primary contenders, he has forced a very public conversation this week about what kind of Democrat should run in 2020. More than two years after the acrimonious primary battle between Bernie Sanders and Mrs Clinton, the divisions over the kind of politics represented by both are still not resolved.
Fresh from a successful House campaign in the midterms which returned dozens of women, non-white and progressive candidates, the liberal wing of the party has a new spring in its step. So far, more leftist candidates have declared, with Elizabeth Warren proposing a tax on high earners, which Mr Schultz, unsurprisingly, opposes.
Ms Harris too has been flaunting her liberal credentials, confirming this week that she supports Medicare for all, a proposal that will delight the more liberal wing of the party, but may not attract centrist suburban voters fearful of losing their healthcare plans.
But others believe the party needs to tack towards the centre in order to guard against another Trump victory. Already this week Joe Biden, who is also mulling a run, highlighted his centrist credentials and willingness to work across the aisle during an appearance in Washington DC.
As the Democratic field begins to take shape, the key question for potential candidates is how they will position themselves within this ideological framework.
Mr Schultz may never formally launch a campaign. But he has started an important conversation about what kind of candidate can take on Mr Trump.