“What I see is that every time Democrats have won since 1960, they’ve won because we had a nominee that excited young people, brought together a new diverse coalition of Americans, and was able to get that victory,” presidential hopeful Julian Castro said on CNN.

The “winning formula” in 2020 “is not to play it safe,” said Castro, a low-polling candidate who served in the Obama administration and is a former mayor of San Antonio, Texas. “It’s not to believe that is if we are just little bit different from Republicans we are going to win.”

Biden, 76, has positioned himself as the moderate, establishment candidate. He leads the Democratic pack in polls and consistently defeats Trump in head-to-head match-ups. But he has committed a series of gaffes on the campaign trail that have raised questions from rivals about his age and stamina.

Notable Gaffes

Last month Biden told a group of Asian and Hispanic voters that “poor kids are just as bright” as white children. And before that he referred to mass shootings in El Paso, Texas, and Dayton, Ohio, as having taken place in Houston and Michigan. He also said the 2018 high school shooting in Parkland, Florida, occurred while he was vice president.

At the end of the second debate, Biden directed viewers to “go to Joe 30330,” flubbing the number to reach his campaign by text, a mistake that went viral.

Senator Amy Klobuchar of Minnesota, another low-polling Democrat who was asked about Biden’s electability on Sunday, answered by talking about her energy.

“I have a lot of energy,” she said on CNN. “I’m someone that never stops working.”

The Biden campaign rejected the criticism.

“Joe Biden’s character, empathy, leadership, and unparalleled record of delivering progressive change make him the best standard bearer for our party to take the fight to Donald Trump and restore the soul of this nation,” campaign spokesman Andrew Bates said in an emailed statement.

“His experience and bold agenda are resonating with the American people because they know who he is and they know his record — which is also why Donald Trump is more afraid of Joe Biden than anyone,” Bates said.

On Friday, another Democratic contender, Ohio Representative Tim Ryan, said Biden is “declining” and couldn’t defeat Trump in what’s expected to be a long, brawling general election campaign.

“I just think Biden is declining,” Ryan said during a phone call. “I don’t think he has the energy. You see it almost daily. And I love the guy.”

On Saturday, Ryan sought to temper those comments.

“I have some concerns with a lot of the candidates,” Ryan said at the New Hampshire Democratic Convention in Manchester. “It was a comment that I made and I’m not backtracking from it, it is a concern you’re hearing from a lot of people.”

A Washington Post-ABC News poll released Sunday found 29% of registered Democrats and Democratic-leaning independents would vote for Biden. The poll of 1,003 adults, conducted Sept. 2-Sept. 5, found 23% supported Senator Bernie Sanders and 11% supported Senator Elizabeth Warren.

Warren is gaining ground on Biden, according to a CBS News/YouGov poll focused on states that will have early primaries and caucuses. Her perceived electability in those states is up, and her implied delegate count is not far below Biden’s.

More must-read stories from Fortune:

—Will gaffes hurt Biden’s chances of a 2020 win? Strategists are divided
—These are the 2020 senate races to watch
—Facebook and Google met with U.S. intelligence about 2020 election
—MSNBC climate change forum will give a voice to those denied the DNC debate stage
—Is Biden preparing to lose in Iowa? His campaign says the caucus isn’t a must-win
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