WASHINGTON — Campaign money from shadowy sources is back this presidential election. At least $4 million of it is flowing to outside groups helping White House hopefuls, making it difficult to trace who’s behind the big bucks. Both parties benefit from money routed through obscure corporations or from nonprofits that don’t disclose donors.

The contributions are a reminder of federal court decisions, including Citizens United, that loosened restrictions in campaign finance laws. That can hide who’s really backing candidates — and what favors or influence could be owed should they get elected.

‘‘Just wait until this year,’’ said billionaire Frank VanderSloot of Idaho. The Associated Press determined that two of his obscure businesses gave a combined $175,000 to a ‘‘super’’ political committee helping Republican presidential candidate Marco Rubio. ‘‘We’re going to send bucketloads. This was teaspoons.’’

American Bridge 21st Century reported more than $1.5 million from its affiliated nonprofit, which doesn’t have to name its donors. American Bridge, which said it used the money to pay for shared expenses like rent and staff, was founded by Hillary Clinton supporter David Brock.

The AP counted more than two dozen groups that each gave at least $50,000 to presidential-aligned super PACs during the last three months of 2015, piecing together property tax documents, public records, and millions of digital campaign finance records. At least half of those were unrecognizable names.

Sheila Krumholz, executive director of the Center for Responsive Politics, said voters need to know the sources of political money so they can evaluate how candidates will behave in office. ‘‘These public officials may be bringing IOUs with them to Washington,’’ she said.

Opaque contributions aren’t new: In 2011, a once-mysterious group gave $1 million to a super PAC supporting GOP presidential candidate Mitt Romney.

The group was formed by an executive at Romney’s old company.

But this year, having no White House incumbent probably means there is more money to go around, especially during a contentious primary season.