Sen. Bernie Sanders remains firm on $3.5 trillion price tag despite grumbles from Sens. Joe Manchin and Kyrsten Sinema

• During an interview with CNN's Anderson Cooper, Sen. Bernie Sanders defended the $3.5 trillion spending bill.
• In recent days, Sens. Manchin and Sinema have doubled down on their opposition to the figure.
• "It's gonna be $3.5 trillion. That's the compromise that's already been made," Sanders said on CNN.

Sen. Bernie Sanders remained firm on the Democrats' proposed $3.5 trillion social spending bill tied to budget reconciliation, during a CNN interview on Tuesday.

"It will be $3.5 trillion. I already compromised... most of the members of the caucus wanted $6 trillion, which is a more adequate number," Sanders told CNN's Anderson Cooper.

"How will the final figure be decided?" Cooper asked Sanders.

"It's gonna be $3.5 trillion. That's the compromise that's already been made," Sanders responded. The cost amounts to $3.5 trillion over 10 years.

Earlier in the day, Sanders told Insider that Democrats are considering $1,000 vouchers for seniors to access expanded Medicare benefits that could form a major part of that spending plan.

Sanders' defense of the bill comes as Sens. Kirsten Sinema and Joe Manchin have doubled down on their opposition to the figure in recent weeks.

On Tuesday, Manchin disagreed with a child tax credit provision include in the social spending package, arguing that recipients should have to work and file taxes in order to receive advance payments. And in late August, Sinema restated her opposition to the price tag.

Every one of the 50 Senate Democrats would have to support the bill, alongside Vice President Kamala Harris's tie-breaking vote, in order to get it to Biden's desk.