“What was a significant downturn for most Republican candidates over the last couple weeks has sort of rebounded a bit,” Holmes said. “All of these competitive races are within the margin of error, and you could have a whole bunch of scenarios play out on Election Day. The options are basically endless.”

Democrats pulled money out of Colorado earlier this month in a sign of confidence in flipping that seat, and Republicans remain heavily favored to regain Alabama. Democrats maintain a clear edge in Arizona, even as some Republicans say their polls show a closer race. Maine is a challenge for Republicans, since Joe Biden is expected to win statewide by a large margin.

Democrat Cal Cunningham still holds a slight edge in North Carolina, despite revelations of his extramarital affair, which gave GOP Sen. Thom Tillis and Republicans new life in a race that had been trending against them. GOP Sen. Joni Ernst and Democrat Theresa Greenfield are locked in a dead heat in Iowa, which is the second-most expensive state. Republicans are pressing their case in Michigan, their only other chance besided Alabama to flip a seat, and Democrats are still spending heavily on defense there, even as public polls show them with a lead.