Mr. Bloomberg — a political independent who has championed left-of-center policies on gun control, immigration and the environment — has approved a plan to pour at least $80 million into the 2018 election, with the bulk of that money going to support Democratic congressional candidates, advisers to Mr. Bloomberg said.

By siding so emphatically with one party, Mr. Bloomberg has the potential to upend the financial dynamics of the midterm campaign, which have appeared to favor Republicans up to this point. Facing intense opposition to President Trump and conservative policies, Republicans have been counting on a strong economy and heavily funded outside groups to give them a political advantage in key races, especially in affluent suburbs where it is expensive to run television ads.

Mr. Bloomberg’s intervention is likely to undermine that financial advantage by bankrolling advertising on television, online and in the mail for Democratic candidates in a dozen or more congressional districts, chiefly in moderate suburban areas where Mr. Trump is unpopular. Democrats need to gain 23 congressional seats to win a majority.