Donald Trump campaigning in North Charleston, S.C., on Feb. 19. (Jabin Botsford/The Washington Post)

This post has been updated.

Super PACs and other independent groups have unleashed a torrent of spending in this year's presidential contest, dumping more than $215 million into ads, mailers, robocalls and other efforts to persuade voters, according to Federal Election Commission filings.

Guess how much has been spent explicitly attacking Republican front-runner Donald Trump?

About $9 million. Just around 4 percent.

The meager investment underscores the uncertainty in GOP establishment circles about how to take on the bombastic real estate developer/reality TV star, known for turning on his critics with vengeance. Many Republican donors have held back from supporting anti-Trump campaigns, worried that the attacks will further embolden him.

"He sort of built up this impression early on that he was impenetrable, that it was throwing good money after bad to take him on," said Republican strategist Katie Packer. "We believe something very different."

In January, Packer launched a super PAC called Our Principles PAC that has vigorously gone after Trump. In just one month, it has risen to be biggest anti-Trump spender of the cycle, plowing $3.8 million into ads that showcase the contradictory positions he has taken over the years.

"We have weakened him considerably," Packer said, noting Trump placed second in Iowa after the group began its assault. "If we had more money and more time, it might have made more of a difference."

Along with Our Principles, Club for Growth Action put $2.7 million into an anti-Trump campaign. Stand for Truth, a super PAC backing Sen. Ted Cruz of Texas, has spent $1.1 million hitting the billionaire businessman, while Keep the Promise I, another pro-Cruz super PAC, has put in another $900,000.

The attacks have ratcheted up in the days leading up to Saturday's GOP primary in South Carolina. Anti-Trump groups spent $3.2 million in that state — a third of the total invested against him to date —  filling the airwaves with ads portraying him as predatory developer and corrupt power broker who sought to buy politicians.

One recent Stand for Truth spot, called "The Trump Show," features a 1999 clip in which the developer declares, "I really believe the Republicans are just too crazy-right."

Packer said she is happy to see other join the fight, but warned that the delay in challenging Trump has been costly.

"There was a calculation by [Jeb] Bush and others that if anyone can get it down to a two-man race, they can beat Trump," she said. "I think that's correct. The problem is if we don’t get it down to a two-man race by March 15 … it becomes harder. The clock is really ticking coming out of South Carolina. We are going to see a winnowing of the field and people are going to have to get very, very serious. We are going to do a lot more. But it's not going to be an easy undertaking at this stage."

Donald Trump's feud with Jeb Bush just keeps getting more heated. (Peter Stevenson/The Washington Post)