"He is," O'Rourke said. "And that cannot be who we are going forward. We've got to be bigger. We've got to be bolder."
Increasingly, other Democrats are denouncing Biden's message, saying he overestimates Republicans' willingness to work with any Democratic president and that his approach to politics falls short of solving the systemic problems facing the country.
The candidates' messages on policy often aren't very far apart. Biden, like other Democratic contenders, talks about building on former President Barack Obama's achievements -- including by implementing a "public option" for health insurance that would allow Americans to buy into Medicare.
At 78 on Inauguration Day 2021, Biden would be the oldest president ever elected. Reflecting the polls, his events have so far drawn generally older crowds than those of other leading 2020 Democratic contenders.
The oldest candidate in the field, Sen. Bernie Sanders of Vermont, and the youngest, South Bend Mayor Pete Buttigieg, both have stronger support among younger voters. And, despite their 40-year age difference, both are making the case for a new generation of politics.
"We're not going to win by playing it safe or promising a return to normal. We are where we are because normal broke," Buttigieg said in Iowa over the weekend, in remarks that were clearly aimed at Biden.
He added that "Democrats can no more promise a return to the '90s than Republicans can deliver on a promise to return us to the '50s."
Sanders told an audience at the California Democratic Party's convention earlier this month that in his view, "We will not beat Donald Trump unless we bring excitement and energy into the campaign. And unless we give millions of working people and young people a reason to vote and a reason to believe that politics is relevant to their lives."
Sanders and Sen. Elizabeth Warren, 69, of Massachusetts are campaigning on ideological change -- an argument that has shown stronger appeal among younger voters.
Polls have shown that Biden's support is concentrated among older voters. In a new Monmouth poll of Nevada likely Democratic voters that showed him with 36% support overall, he was backed by 47% of those older than 65, 35% of those between 50 and 64, and 29% of those 18 to 49.
Sanders' campaign views those in Iowa between ages 17 (they're allowed to caucus as long as they'll be 18 by Election Day in November 2020) and 21 as an opportunity to grow his support from 2016, senior Iowa adviser Pete d'Alessandro said.
"Those are both young people, which we appeal to, but they're also new caucus-goers, because they didn't caucus last time. So they're part of the new people that we're bringing into it and, since we do well with them, that's part of expanding the universe" of Sanders supporters, he said.
O'Rourke's comments were a major departure from his previous praise for Biden, who he had urged to run for president. O'Rourke pointed to Biden's recent reversal on whether the federal government should pay for abortions, his mixed messaging on China and his vote to authorize the war in Iraq -- which Biden since 2005 has called a mistake -- saying, "You've got to ask yourself where Joe Biden is on issues that are most important to you."
"I'm not exactly sure what he believes or what he should apologize for. I only know that this country should be able to do far better," O'Rourke said.
Sen. Michael Bennet of Colorado, when asked on PBS last week whether Biden represents the future, said: "No, I don't think so."
He also downplayed Biden's status as the front-runner in national and early-voting state polls as a product of the former vice president being more widely known than other Democratic candidates.
"It's time for a new generation of leadership," Bennet said. "And I'm not surprised that he's leading in the polls, because he has more name recognition, certainly, than I do."
CNN's Gregory Krieg contributed to this report.