A year later, Harris is out of the race, and Biden and Sanders are front-runners for the Democratic nomination. Both have overcome speed bumps in their campaigns, including a heart attack for Sanders, refined the rationales for their candidacy, and maintained the support of key Democratic constituencies — black voters for Biden and younger voters for Sanders.

The durability of two white men in their late 70s has surprised many Democrats and prompted questions about representation and electability in a party that will count on high turnout among women, minorities and young voters in November's general election faceoff against President Donald Trump. Biden, 77, and Sanders, 78, would each be the oldest president in American history on Inauguration Day.

"We like to pride ourselves on not being the party of old, white men," said Sue Dvorsky, the former chairwoman of the Iowa Democratic Party, who endorsed Harris in the campaign. "But there is something to the fact that these two, they're known quantities in a time when everything is so utterly unknown."

Biden and Sanders still face stiff competition in the early voting states that could block their paths to the nomination. They're locked in tight, four-way races in Iowa and New Hampshire with Massachusetts Sen. Elizabeth Warren and Pete Buttigieg, the former mayor of South Bend, Indiana. A win for any of the candidates in the first two contests would give their campaigns a crucial jolt.

Yet Biden and Sanders, two ideological foes, have increasingly been tangling in ways that suggest they expect to be the last two standing in the Democratic primary.

Sanders has questioned Biden's judgment in voting for the Iraq war in 2003, while the former vice president has cast the Vermont senator's government-run health care proposals as risky and astronomically expensive. On Saturday, Biden called for Sanders to disavow a misleading video a Sanders aide put out suggesting the former vice president endorsed Republicans' calls for cutting Social Security and Medicare.

Other campaigns are still grappling with how best to cut into Biden and Sanders' support. Buttigieg, another more moderate candidate, could benefit from a Biden dip in the early states, but he has made no measurable progress with voters. Warren and Sanders, two progressive favorites, have seemed destined for an inevitable clash, but both tried to back away from the first rift that emerged last week: a disagreement over whether Sanders told Warren in a private meeting that a woman can't beat Trump.

The prospect of a Biden-Sanders faceoff deep in the primary isn't what many Democrats would have predicted a year ago, as the party's primary filled out with a diverse cast of senators, governors and rising political stars.

At the time, Democratic strategists openly speculated that Biden, especially, in his third bid for president was bound to stumble due to his frequent verbal miscues and a resume from 40 years in politics that can appear out of line with a party shifting to the left. Admirers who worked alongside him in the Obama White House privately worried that a bruising campaign would erase the goodwill he built up over eight years as vice president, particularly after he grieved publicly over the 2015 death of his son, Beau.

Some rivals tried to pounce quickly, with Harris launching an aggressive and deeply personal debate stage attack on Biden's opposition in the 1970s to federally mandated school busing. The moment gave Harris a sudden boost and appeared to raise questions about Biden's viability. But it proved to be a sugar high for Harris, and Biden quickly rebounded.

Sanders has also faced questions about whether he could replicate the enthusiasm of his 2016 campaign, when he split the vote in the Iowa caucuses with Hillary Clinton and raised eye-popping sums of money from small donors that allowed him to challenge her to the end of the primary campaign. Without a head-to-head race against a flawed opponent like Clinton, and with Warren, another progressive star, in the race, some Democrats posited Sanders would struggle to replicate his past success.

But Lily Adams, who served as Harris' communications director, said of both Biden and Sanders: "Once the summer was over, it was clear both of them had durability."

Ed Rendell, the former Pennsylvania governor and a Biden supporter, said other campaigns focused too much on Biden's and Sanders' age, without realizing that their years in the public eye came with an upside.

"When people have seen a Joe Biden or a Bernie Sanders in action for 10, 20, 30 years, their opinions of them, if they liked them, become baked in," Rendell said.

Sanders has led the Democratic field in fundraising, pulling in $34.5 million in the fourth quarter — all raised after an October heart attack that pulled him off the campaign trail for several days. Sanders has kept up a robust campaign schedule ever since, with even his rivals commenting on the senator's energy.

"Bypass surgery generally makes the patient healthier than he or she was before," Rendell said. "That's almost a plus for Bernie. ... Just look at him."

With so many once-promising rivals out of the race as the Iowa caucuses near, both campaigns are relishing, so far, proving doubters wrong.

"You know, a year ago our friend from Texas was going to be the president — Beto," said Sanders' top political adviser, Jeff Weaver, referring to former Texas Rep. Beto O'Rourke. "Kamala Harris was going to be president, and on and on and on."

Biden aide Symone Sanders blamed "the media and the pundit class" for underestimating her boss. Voters, she added, "consistently said they knew who Joe Biden was."

With the first primary votes yet to be cast, Democratic sentiment could still quickly turn against Biden and Sanders. The senator, who struggled in the 2016 campaign with minority voters, faces the same questions about his ability to appeal to black voters as the race heads to South Carolina and other Southern states. Biden must prove that he cannot just win, but also energize young Democrats who vote in lower numbers in general elections and could make the difference in a close race against Trump.

But already, Biden and Sanders have won grudging respect from some former doubters, who say the two septuagenarians have proven they've learned a thing or two from their previous White House bids.

"It is remarkably, remarkably hard to run for president. And these two guys have done it before," Dvorsky said. "If you've done it once, you're not starting from scratch. You're starting already on second base."