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By Daniela Altimari and Mary Ellen McIntire

Democrats are sounding the alarm on mystery super PACs with ties to Republicans that are meddling in Democratic primaries.

“Clearly Republicans know they can’t win in the midterms,” said Washington Rep. Suzan DelBene, who chairs the Democratic Congressional Campaign Committee, “so they are reverting to trying to rig the system … they are playing in primaries and trying to do so in a secretive way.”

A pop-up super PAC called Lead Left PAC and others like it have poured money into a handful of Democratic primaries, in most cases propping up the more progressive contender, who Republicans believe will be easier to beat in swing districts.