Donald Trump. REUTERS/Jonathan Ernst Republican presidential frontrunner Donald Trump suggested Monday that the billionaire Ricketts family should "be careful."

"I hear the Rickets family, who own the Chicago Cubs, are secretly spending $'s against me," Trump tweeted. "They better be careful, they have a lot to hide!"

It's not a secret, but one member of the Ricketts family has indeed been quietly donating money to the anti-Trump super PAC called Our Principles PAC. Marlene Ricketts gave $3 million.

Marlene is the wife of TD Ameritrade founder J. Joe Ricketts. USA Today noted that she has also given money to super PACs backing the presidential campaigns of former Gov. Rick Perry of Texas, Sen. Marco Rubio of Florida, former Gov. Jeb Bush of Florida, Gov. Chris Christie of New Jersey, Sen. Ted Cruz of Texas, and Sen. Lindsey Graham of South Carolina.

Those donations of $10,000 each were made last year. Perry, Bush, Christie, and Graham have since dropped out of the Republican race.

Our Principles PAC has attacked Trump on his positions on universal healthcare, gun control, and abortion. There are more than a dozen anti-Trump ads on its YouTube page.

Here's one such ad: