The most liberal Democrats
The most liberal Democrats are probably less likely to vote against Biden proposals compared to their conservative counterparts for two reasons. First, they will have no credible claim that voting for something that Biden favors would hurt them electorally, since more liberal members predominantly represent safely Democratic districts and states. Secondly, it’s hard for a member of Congress to tank a bill that he or she mostly agrees with but thinks doesn’t go far enough.
That said, progressives in the House did block some of House Speaker Nancy Pelosi’s proposals in 2019 and 2020 and are likely to act as a check against Biden going too far to the right in the view of the party’s progressives.
I would expect Sens. Bernie Sanders and Elizabeth Warren (the most liberal senator according to DW-Nominate) to be the most prominent voices in the Senate trying to push legislation to the left. In the House, expect to see new members Jamaal Bowman of New York and Cori Bush of Missouri essentially become new members of “The Squad” (Alexandria Ocasio-Cortez of New York, Ilhan Omar of Minnesota, Ayanna Pressley of Massachusetts and Rashida Tlaib of Michigan) and that bloc to be the left flank in the House. Another very liberal House member to watch is Pramila Jayapal of Washington, the leader of the Congressional Progressive Caucus, which includes about 100 members.