The Club for Growth was the first conservative group to strap on "Stop Trump" spurs, with a summer 2015 ad barrage in Iowa. When polls showed Trump slipping, and Ben Carson gaining at his expense, the Club took credit. "[Club for Growth] Action’s ads have exposed the truth about his history of support for higher taxes, and his advocacy for single-payer health care, eminent domain, and bailouts," David McIntosh, the Club's president, said in October. "That truth has erased Trump’s lead."

Five months and 20 Trump state wins later, the Club remains at the forefront, reporting $4 million raised last month to pay for stop-Trump ads. But the sort of ads being run now demonstrate the difficulty of beating Trump. The Club's latest spot, running in Utah until Tuesday's caucuses, is at least the fourth that tries to weaponize a "60 Minutes" interview from last year in which Trump said "the government" would pay for health-care plans that covered the poorest Americans.

That clip previously made it onto TV screens courtesy of the Our Principles PAC.

Before that, "Trumpcare" was pilloried by the pro-Ted Cruz PAC Keep the Promise I.

And before that, the Club ran a short web ad to make voters aware of Trump's health-care quote.

In each case, the Trump quote was truncated to make him sound like a proponent of a much less conservative idea. (An exchange with Scott Pelley, which makes it clear Trump was referring only to Americans who could not afford their own coverage, is always snipped out.) Yet there's no evidence that pounding Trump over this quote has hurt his rise.

The reason, according to someone familiar with stop-Trump polling, is that the mogul's voters simply don't disagree strongly with the candidate. In one poll conducted in the past week, potential Trump voters were asked whether they would bail on him if convinced that he would fail to repeal the Affordable Care Act. Less than a third said they would.

The Club is still studying how to attack Trump in Wisconsin, where the April 5 primary is seen as the latest "last chance" for conservatives to rally behind a Trump opponent.