Democrats were already enormous underdogs to retain the Senate at the beginning of this whole election cycle because of how unfavorable the map was this cycle. The fact that Ted Cruz and Rick Scott may even have to sweat bullets- even though they will still likely win their respective races- is a testament to how deeply unpopular the Republican Party is even in some red states.
The realistic goal for the Democrats- one that appears to be very much attainable, if not more probable than not at present- is to retake the House and retain the White House and keep the Senate at a razor thin Republican margin such that something like, say, a Susan Collins being offered a position in the Harris administration could create a 50/50 tie in the Senate with Tim Walz being able to cast tie-breaking votes if necessary.