What is just as bad for the Democrats as falling poll numbers is the trend in third party voters switching to the GOP. This is what happended in 2016, when Johnson polled at close to 10%, but dropped to just over 3% on election night (many of those voting for Trump). As a case in point, Pennsylvanina was one of the closest races in 2016 (Trump winning by a mere 0.7%). One month before the election, the polls were Clinton 47.7%, Trump 43.8%, and Johnson 7.2%. The final tally was Clinton 47.5%, Trump 48.2%, and Johnson 2.4%. Almost all of Trumps support came from voters opting to drop their support for the third party candidate. FYI, the current polling average (last 4 polls) in PA are Fetterman 46.8%, Oz 46.3%, 3.7% third party, and 3.2% undecided. It is not looking good for the Dems in the Keystone state.
Many of the other senate races have a third party candidate polling at 2% or above (NH, AZ, PA, GA, NV, and NC) and/or a significant number of undecideds. That may not sound like much, but in these close races, it might make the difference, and in Georgia, it is likely enough to force a December runoff (again).
This is the main reason why 538 was so far off in 2016, predicting Clinton to win PA, MI, NC, WI, and FL and the presidency. They correctly picked Biden in 2020, but that was largely due to higher polling margins for Biden prior to election day. The 538 polling margin in the 10 closest states favored Biden by 3.3%. The actual results favored Trump by 1.3%, for a 4.6% error - larger than the posted margin of error by most pollster (3-4%). Historically, the largest polling biases in the Dems favor have been Florida, Iowa, Michigan, Ohio, Pennsylvania, and Wisconsin (all but Michigan have a senate race this year). Consequently, any state in which the Democratic candidate is not polling over 50% should be considered a toss-up, even New Hampshire, where Hassan is polling at around 49%. While I think she will eventually win out, it will likely be close.