Supporters of Democratic presidential front-runner Hillary Clinton listen as she speaks at a rally at Snug Harbor's Great Hall in Staten Island, N.Y. on April 17. (Melina Mara/The Washington Post)

The super PAC supporting Hillary Clinton's presidential campaign is preparing to spend tens of millions of dollars online to reach potential supporters in seven swing states beginning this summer -- the largest ad buy of its kind this cycle, according to the group.

Priorities USA is reserving $35 million in online advertising in Colorado, Florida, Iowa, Nevada, New Hampshire, Ohio and Virginia, targeting millennials, Hispanics, African Americans and women who watch videos or use online streaming services. The group plans to make the formal announcement Monday.

Spokesman Justin Barasky said the buy should help "level the playing field" for Democrats as they try to ward off attack ads by Republicans and conservative groups against Clinton in the general-election campaign.

The $35 million will be spent only on video advertising, the most valuable kind of online advertising available. The buy will build on a $90 million television advertising campaign previously announced for the same seven states.

These days, spending money to find voters online is a must. A quarter of millennials don't watch television on broadcast or cable channels and 66 percent of Hispanics use their mobile devices as their primary source of consuming media, according to statistics Barasky cited.

No other super PAC has announced plans for as ambitious an advertising campaign, he added. As with most groups, Priorities USA doesn't rule out expanding the buy as the campaign progresses.

Priorities USA has plenty of money to spend -- and is poised to raise much more.

So far this 2016 cycle, the group has raised at least $104 million. There was $44.5 million in the bank at the beginning of March and an additional $49 million in commitments, according to campaign spending reports. Most of that money came from Fred Eychaner, a Chicago media executive and major Democratic donor, but also included donations from supporters including actress and activist Jane Fonda.

The group initially didn't plan to start spending on advertising until the general-election season, but jumped in early in February to help Clinton win over black voters in the South as she faced a tougher-than-anticipated challenge from Sen. Bernie Sanders (Vt.).