The list of swing states included in Mr. Bloomberg’s anti-Trump campaign would most likely grow in the future, Mr. Wolfson said.
Mr. Bloomberg also spent heavily in the 2018 midterms, pouring about $100 million into Democratic efforts, particularly to win control of Congress. Democrats won 21 of the 24 House races he invested in.
The new anti-Trump campaign by Mr. Bloomberg is the second major infusion of Democratic money into online advertising in two weeks. A new progressive organization, Acronym, announced earlier in November that it and an affiliated PAC were investing $75 million in online ads in battleground states.
That group, advised by David Plouffe, who managed Barack Obama’s first presidential campaign, was focused on the same four states as Mr. Bloomberg, plus North Carolina. Mr. Plouffe said Acronym’s goal was to ensure that whoever emerged as the party’s nominee was not in a “hole too steep to dig out of.”
In addition, Priorities USA, the leading Democratic super PAC in the 2016 race, has said it plans to spent $100 million, including on digital ads, its analytics program and on television, through the convention. The group held a briefing this week on its anti-Trump program and said that, since late July, its PAC had actually outspent Mr. Trump’s campaign in the four battlegrounds where it was advertising: Florida, Michigan, Pennsylvania and Wisconsin. The group noted that much of Mr. Trump’s spending had been targeting new donors versus trying to persuade swing voters.
Priorities USA also announced that it would expand to Arizona, one of the states that Mr. Bloomberg is targeting, with a $2 million digital buy.
Mr. Bloomberg has flirted with running for president before — including as an independent in 2016 — but he has taken concrete steps toward a Democratic primary campaign in recent days, including submitting paperwork to appear on the ballot in Alabama and Arkansas, the states with the earliest filing deadlines.