Sen. Bernie Sanders (I-Vermont) was once President Joe Biden’s primary rival in 2020. Now, the influential senator is calling on Biden — whom he has already endorsed in 2024 —to communicate a bold message to voters still on the fence about his candidacy. A recent Bloomberg/Morning Consult poll found that Biden is still trailing former President Donald Trump in the seven swing states most likely to decide the November election — Arizona, Georgia, Michigan, Nevada, North Carolina, Pennsylvania and Wisconsin. The chief concern about Biden among holdout voters in those states is his age, according…