Campaign money from shadowy sources is back this election.

Hillary Clinton speaks during a campaign event at Winnacunnet High School in Hampton, New Hampshire. (Photo by Justin Sullivan/Getty Images)

More than $4 million of it channeled to outside groups helping presidential candidates has come from unknown or masked donors.

Super PAC's helping hopefuls like Marco Rubio and Hillary Clinton received big checks recently from obscure corporations or from nonprofits that don't have to disclose their donors' names.

It's a reminder of federal court decisions in recent years loosening prior restrictions in campaign finance laws.

That has made it difficult at times to tell who is really backing candidates, and what favors or influence could be owed should they get elected.