Priorities USA Action, the leading super PAC backing presumptive Democratic presidential nominee Hillary Clinton, collected more than $12 million in May in its best fundraising month yet. The haul was driven by a handful of wealthy liberal donors and labor unions that plowed six- and seven-figure checks into the group.

The largest contribution in May came from Chicago media executive Fred Eychaner, who gave the super PAC $3 million. Eychaner had donated $2 million to the super PAC in February. Hedge fund manager Donald Sussman gave $2 million, bringing his total contribution to $3.5 million. Among the big labor union donations was $1 million from the American Federation of Teachers and another $1 million from the Laborers' International Union, which has given a total of $3 million.

Priorities has raised $88.7 million in all and has another $45 million in commitments, officials said. The group is in the midst of a $20 million pre-convention TV advertising campaign in swing states, releasing a new spot Sunday featuring some of GOP contender Donald Trump's most provocative comments on foreign policy. As of the end of May, the super PAC had nearly $52 million in the bank.

The organized and well-funded push for Clinton contrasts sharply with the chaos that surrounds the groups supporting Trump. At least five super PACs are in the mix, creating confusion among donors unsure where to put their money.

One of the first groups on the scene, Great America PAC, has raised just $3.5 million since forming in February. The super PAC rolled out a new ad Monday spotlighting the massacre in Orlando and featuring a former Navy SEAL who touts Trump as "willing to make tough choices to protect America." The spot is part of a $700,000 TV campaign set to air nationally and in key states over the next several weeks.

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