Organized labor has poured about $45 million into pro-Democratic super PACs in the current election cycle, according to data reported to the Federal Election Commission. But only a small portion of that money has been spent, meaning that Republican candidates likely will face a heavy barrage of attack ads in the fall.

Federal filings show, for example, that unions have donated about $10.7 million to Priorities USA Action, the top Democrat-aligned super PAC, accounting for about one-sixth of the group's funds. That exceeds the donations by deep-pocketed liberal donors such as George Soros ($7 million), Univision owner Haim Saban ($3.5 million), and Herbert and Marion Sandler ($2.5 million), according to the Center For Responsive Politics.

In the last presidential election cycle, unions donated $13.4 million overall to Priorities USA Action. With six months remaining this year's election, the unions could easily top that figure before the year's end.

Super PACs are campaign organizations that, unlike traditional political action committees, can raise unlimited amounts from individuals, corporations and labor unions, and are not limited in the amounts they can spend to back or oppose a particular candidate. Legally, they are obligated to be separate from the candidate's campaign, though most are run by people with close ties to the candidate or party the super PAC is aiding. Donations to super PACs are separate from direct donations to candidates' campaigns, which are capped by law.

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