Sen. Bernie Sanders of Vermont has not ruled out running for president in 2024 if Joe Biden does not run again.

"In the event of an open 2024 Democratic presidential primary, Sen. Sanders has not ruled out another run for president, so we advise that you answer any questions about 2024 with that in mind," said Sanders' former campaign manager Faiz Shakir, according to a copy of a memo first obtained by the Washington Post.

Sanders, who also ran in 2016, has maintained strong support from the more liberal base of the Democratic party due to his stances on taxes, wages, lower college tuition and addressing climate change.

"As campaigning heats up in states across the country, your political opponents and their corporate-aligned allies will try to make you feel defensive about Sen. Bernie Sanders’ support for your candidate," Shakir wrote.

"Our advice is to embrace the attacks,” he added.

The memo notes that Sanders has not made concessions to past platforms.

"Sen. Sanders is putting forward an extremely popular vision for the Democratic Party that will win back critical support that we have lost," Shakir said.

"In fact, Bernie wants to build power for the working class and take on the corporate socialism that our political system currently favors," he said.