WASHINGTON — Hillary Clinton told a cheering crowd at her largest rally so far that “the endless flow of secret, unaccountable money” must be stopped. Two weeks later, the main super PAC backing her bid for the Democratic presidential nomination accepted a $1 million contribution that cannot be traced.

The seven-figure donation, made June 29 to the pro-Clinton Priorities USA Action, came from another super political action committee, called Fair Share Action.

Its two lone contributors are Fair Share Inc. and Environment America Inc., according to records filed with Federal Election Commission.

Those two groups are nonprofits that are not legally required to reveal information about their donors. Such contributions are sometimes called “dark money” by advocates for stricter campaign finance rules.

“This appears to be an out-and-out laundering operation designed to keep secret from the public the original source of the funds given to the super PAC, which is required to disclose its contributors,” said Fred Wertheimer, director of one such group, the Washington-based Democracy 21.

Wertheimer urged Priorities to return the money and said that Clinton should demand that the super PAC “publicly disclose all of the original sources of money” of any contribution it receives.

It’s a suggestion rejected by Priorities USA, whose spokesman, Peter Kauffmann, said the group is “playing by the rules.”

“In the face of a billion dollar onslaught by right wing groups, there is too much at stake for everyday Americans for Democratic groups to unilaterally disarm,” he said.

Priorities USA raised about $15.6 million in the first six months of the year.

While another Democratic competitor, Vermont Sen. Bernie Sanders, has rejected the support of super PACs altogether, Clinton has been plain that she needs big-money help.

“We’re going to have to do what we can in this election to make sure that we’re not swamped by money on the other side,” she said last month.

Several of the Republican candidates for president also have nonprofits dedicated to helping their candidacies.