Republican presidential candidate Donald Trump fell to second place in Iowa in a new poll released Tuesday from fiscally conservative super PAC Club For Growth, which has been openly campaigning against the real estate mogul, according to the Hill. Ben Carson took first place in the poll with 21 percent support, followed by Trump at 16 percent.

In other polls, Trump's support has dropped since the Sept. 16 GOP debate in California, but Club For Growth's report is the first since late July that did not find Trump to be the GOP front-runner in the presidential race.

The Club for Growth survey interviewed 500 Republicans planning to go to caucus and was conducted by GOP polling firm Basswood Research, Oct. 3-4. The poll has a 4.4 percentage point margin of error.

Club for Growth attributed Trump's fall to their $1 million purchase of attack ads in Iowa calling attention to his past as a Democrat and support of left-leaning policies. Based in Washington, D.C., the super PAC is the "leading free-enterprise advocacy group in the nation," according to its website.

“Donald Trump is not a conservative, and that message is getting through in Iowa,” Club for Growth President David McIntosh said in a statement. “Club for Growth Action can’t take full credit. Trump’s stumbling and substance-free debate performance surely hurt him; but CFG Action’s ads have exposed the truth about his history of support for higher taxes, and his advocacy for single-payer healthcare, eminent domain and bailouts. That truth has erased Trump’s lead.”

Trump threatened to sue Club For Growth last month when his campaign sent the group a "cease and desist letter" for misrepresenting his "views on taxes by taking an old statement that he would like to hike taxes across the board on the 'superwealthy.' " But the letter did not use the correct legal jargon to technically be considered a cease and desist letter, according to the Hill.