“Within 15 minutes of the sad news, you saw truly organic movement,” said David Keith, who is managing Dr. Gross’s bid to oust Senator Dan Sullivan, a Republican.

From Alaska to Maine to North and South Carolina, Democratic strategists working on Senate campaigns described a spontaneous outpouring of donations the likes of which they had never seen, allowing Democrats the financial freedom to broaden the map of pickup opportunities, or press their financial advantage in top battlegrounds already saturated with advertising.

By Monday, Democratic contributors had given more than $160 million online through ActBlue, the leading site for processing digital donations. ActBlue broke one record after another — its biggest hour in 16 years, its busiest day, its busiest weekend — after Justice Ginsburg’s death, with an estimated tens of millions of dollars going toward efforts to retake the Senate, where the acrimonious confirmation fight to replace Justice Ginsburg will occur.