Donald Trump speaks at a campaign rally at the Georgia World Congress Center in Atlanta on Sunday. REUTERS/Tami Chappell

Donald Trump went after the family that owns the Chicago Cubs on Monday after campaign finance filings revealed that its matriarch, Marlene Ricketts, provided $3 million in seed money for a super PAC running attack ads against him.

Trump tweeted that the Ricketts family "be careful, they have a lot to hide!" He claimed that they were "secretly" spending money against him. In fact, Marlene Ricketts's donation to the Our Principles PAC was reported in public documents filed to the Federal Election Commission.

A spokesman for the Ricketts family declined to comment.

Marlene Ricketts is married to Joe Ricketts, the billionaire founder of T.D. Ameritrade. One of their four children, Pete, is the governor of Nebraska. Another son, Todd, is co-owner of the Chicago Cubs and was a top fundraiser for the short-lived presidential campaign of Wisconsin Gov. Scott Walker.

In January, Marlene Ricketts help jump start the newly-formed Our Principles PAC, giving the group $3 million in three separate contributions.

The super PAC also received $7,500 from Illinois businessman Richard Uihlein. In just one month, Our Principles has emerged as the biggest anti-Trump spender of the cycle, plowing more than $3.8 million into ads that showcase contradictory positions he has taken over the years.

[Super PACs have spent more than $215 million on the presidential race. Guess how much against Trump?]

Katie Packer, the Republican strategist leading Our Principles PAC, said Trump's tweet was an effort to silence his critics.

"It's what we've come to expect from Donald Trump," she said. "He doesn't have any answers for anything we've put up on TrumpQuestions.com, so he tries to bully and intimidate and threaten people. It’s designed to scare people out of contributing, but I don’t think any of our donors were deluded into thinking they wouldn’t become targets of Trump. They knew this was coming."

Trump spokeswoman Hope Hicks fired back in a statement, saying: "That is rich. Someone spending millions of dollars of other people's money to disparage Mr. Trump characterizes his defense as bullying?"

"Mr. Trump will continue to tell it like it is, especially, where out of touch establishment elites are concerned," she added.

For her part, Packer said Our Principles is not backing down, saying there is a spate of new interest from prospective donors in the wake of Trump's victory in the South Carolina primary.

"I think people understand the threat is very serious," she said, adding, "We expect we’ll be able to have a serious campaign in the March 1 states."

Anu Narayanswamy contributed to this report.