Well, the conventional wisdom suggests that even if Democrats take control of the Senate, the narrow victory will mean that a Biden Administration will be constrained in setting a new agenda.
The budget reconciliation process, which can be used by the smallest majority, allows any administration to pass large scale programs with a single vote advantage in the Senate, as long as the legislation is deficit neutral … kind of. The Trump Administration, for instance, used the reconciliation process to pass its $1.5 trillion tax cut and managed, somehow, to avoid the requirement that it not increase the deficit.
If, however, the Democrats manage to take the Senate, I would bet that a President Biden and Vice-President Harris, who represents the tie-breaking vote in the Senate, may not feel as constrained as many currently suggest.