DES MOINES, Iowa (AP) — When Sen. Kamala Harris’ advisers assessed the Democratic primary field early in her campaign, they viewed Joe Biden as headed for an inevitable collapse and Bernie Sanders as unlikely to recapture the magic of his 2016 campaign.

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.”