Sen. Bernie Sanders (I-Vt.) is privately expressing concerns about Joe Biden’s presidential campaign, according to three people with knowledge of the conversations, and is urging Biden’s team to intensify its focus on pocketbook issues and appeals to liberal voters.

Sanders, the runner-up to Biden in the Democratic primaries, has told associates that Biden is at serious risk of coming up short in the November election if he continues his vaguer, more centrist approach, according to the people, who spoke on the condition of anonymity to describe sensitive talks.

The senator has identified several specific changes he’d like to see, saying Biden should talk more about health care and about his economic plans, and should campaign more with figures popular among young liberals, such as Rep. Alexandria Ocasio-Cortez (D-N.Y.).

Asked for comment, Sanders’s team provided a statement from Faiz Shakir, the senator’s campaign manager in the presidential race, saying that Sanders is “working as hard as he can” to get Biden elected but has advised some strategic adjustments.

“Senator Sanders is confident that Joe Biden is in a very strong position to win this election, but nevertheless feels there are areas the campaign can continue to improve upon,” Shakir said. “He has been in direct contact with the Biden team and has urged them to put more emphasis on how they will raise wages, create millions of good paying jobs, lower the cost of prescription drugs and expand health care coverage.”

Shakir said Sanders “also thinks that a stronger outreach to young people, the Latino community and the progressive movement will be of real help to the campaign.”

The Biden campaign declined to comment.

Sanders on Sunday confirmed many of these details, highlighting the specific changes he would like to see Biden’s team make, even after first denying Saturday’s report. “Of course they’re not true. I mean look, what I have said privately is what I have said publicly, and that is I think Biden’s in an excellent position to win this election,” Sanders said. “But, I think we’ve got to do more as a campaign than just go after Trump.”

He then urged Biden to talk more about the economy, specifically highlighting what he will do to raise the minimum wage, create jobs and expand health care and reach out “more aggressively to grass-roots Latino organizations.”

Sanders led a surging liberal faction during the Democratic primaries and scored early successes in Iowa, New Hampshire and Nevada before ultimately falling short. His critique of Biden’s approach reflects his status as a longtime stalwart of the party’s left and a self-described democratic socialist.

But it is rare for such a prominent party figure to repeatedly voice private criticisms of the party’s nominee and acknowledge them publicly, especially in the campaign’s final stretch. Sanders’s decision to do so suggests the ongoing frustration among liberals, who urgently want Biden to defeat President Trump but are upset that he has taken a relatively centrist path.

Biden is determined not to play into attacks from Trump seeking to cast him as a radical or a socialist. The nominee has distanced himself from elements in his party calling for defunding the police, implementing a single-payer health plan and banning hydraulic fracturing, or fracking. Sanders supports the latter two policies.

Still, Sanders has worked hard publicly in support of Biden, and Democrats are eager to avoid a repeat of the division between the senator and Hillary Clinton that hurt the party in 2016. On Saturday, Sanders hosted a virtual town hall — his eighth such event — in which he urged Michigan voters to support Biden. But he also said it was important to continue fighting for progressive values, at times acknowledging that he did not think the Democratic nominee was going far enough on those issues.

But Sanders contends that Democrats have the best chance of winning if they stress economic populism, those close to him said, rather than if they embrace a sole strategy of attacking Trump and avoiding hot-button issues.

Until now, there had been few outward signs of discord between Biden and Sanders. Shortly after the senator ended his campaign in April, he promptly endorsed Biden, offering unequivocal approval.

Associates of both men say they personally like each other, having been Senate colleagues. After Biden emerged as the presumptive presidential nominee, the two formed a number of task forces, made up of allies of both men, that crafted policy recommendations on health care, climate change and other topics.

In some ways, Biden has moved closer to Sanders’s brand of populism as left-leaning activism has surged inside and outside the Democratic Party. He has spoken of a Franklin D. Roosevelt-style presidency if he wins and urged sweeping change to combat the novel coronavirus, racism and other challenges.

But Sanders has come to worry about the Biden campaign’s prospects, even as the Democratic nominee leads Trump in national polls, associates said. Surveys in some potentially pivotal states show a closer race between Biden and Trump, stoking nervousness among Democrats still traumatized by Clinton’s 2016 defeat.

The people familiar with Sanders’s private conversations said he has expressed a sentiment that the liberal, millennial slice of the party has not received the attention it merits. As a candidate, Sanders drew big crowds of hundreds — sometimes thousands — of young, enthusiastic people with left-leaning views.

Another Sanders concern, according to one of the people, is that the Biden campaign has kept its distance from some of the marquee surrogates who campaigned for Sanders and helped him attract a large following. Ocasio-Cortez, for example, has not campaigned closely with Biden.

As a candidate, Sanders frequently emphasized his economic plans, which were geared toward curtailing wealthy and powerful interests and championing working-class people. Biden has recently been touting his “Build Back Better” plan, which calls for immense new investments in American jobs and industry.

And in questioning Biden’s outreach to Latino voters, among whom Sanders showed strength in the primaries, the senator is touching on a topic that is increasingly on the minds of Democratic leaders.

Polls have shown Biden leading Trump among Latinos but not as widely as many Democrats hoped. As a result, fretful discussions are underway in the party about Biden’s standing with Latino voters in battleground states such as Florida, Arizona, North Carolina and Pennsylvania, where Latinos could play a pivotal role in the outcome.

Amy B Wang contributed to this report.