A well-funded group of anti-Trump conservatives has sent its donors a remarkably candid memo that reveals how resilient former President Donald J. Trump has been against millions of dollars of negative ads the group deployed against him in two early-voting states.

The political action committee, called Win It Back, has close ties to the influential fiscally conservative group Club for Growth. It has already spent more than $4 million trying to lower Mr. Trump’s support among Republican voters in Iowa and nearly $2 million more trying to damage him in South Carolina.

But in the memo — dated Thursday and obtained by The New York Times — the head of Win It Back PAC, David McIntosh, acknowledges to donors that after extensive testing of more than 40 anti-Trump television ads, “all attempts to undermine his conservative credentials on specific issues were ineffective.”

The memo will provide little reassurance to the rest of the field of Mr. Trump’s Republican rivals that there is any elusive message out there that can work to deflate his support.

“Even when you show video to Republican primary voters — with complete context — of President Trump saying something otherwise objectionable to primary voters, they find a way to rationalize and dismiss it,” Mr. McIntosh states in the “key learnings” section of the memo.

“Every traditional postproduction ad attacking President Trump either backfired or produced no impact on his ballot support and favorability,” Mr. McIntosh adds. “This includes ads that primarily feature video of him saying liberal or stupid comments from his own mouth.”

For the polling underpinning its analysis, Win It Back used WPA Intelligence — a firm that also works for the super PAC supporting Mr. Trump’s chief rival in the race for the presidential nomination, Gov. Ron DeSantis of Florida.

Examples of “failed” ads cited in the memo included attacks on Mr. Trump’s “handling of the pandemic, promotion of vaccines, praise of Dr. Fauci, insane government spending, failure to build the wall, recent attacks on pro-life legislation, refusal to fight woke issues, openness to gun control, and many others.” (Dr. Anthony S. Fauci led the national response to the Covid pandemic.)

The list of failed attacks is notable because it includes many of the arguments that Mr. DeSantis has tried against Mr. Trump. The former president leads Mr. DeSantis by more than 40 points in national polls and by around 30 points in Iowa, where Mr. DeSantis’s team believes he has the best shot of defeating Mr. Trump. Mr. McIntosh, a former Indiana congressman who co-founded the Club for Growth and the Federalist Society, makes it clear in the memo that any anti-Trump messages need to be delivered with kid gloves. That might explain why Mr. DeSantis’s super PAC, Never Back Down, has treated Mr. Trump gingerly, even in ads meant to contrast his character and his record unfavorably against Mr. DeSantis’s accomplishments.

“Broadly acceptable messages against President Trump with Republican primary voters that do not produce a meaningful backlash include sharing concerns about his ability to beat President Biden, expressions of Trump fatigue due to the distractions he creates and the polarization of the country, as well as his pattern of attacking conservative leaders for self-interested reasons,” Mr. McIntosh writes in the memo.

“It is essential to disarm the viewer at the opening of the ad by establishing that the person being interviewed on camera is a Republican who previously supported President Trump,” he adds, “otherwise, the viewer will automatically put their guard up, assuming the messenger is just another Trump-hater whose opinion should be summarily dismissed.”

The polling conducted for Win It Back showed diminishing returns for the anti-Trump messaging and emphasized that Mr. Trump benefited from the fact that his rivals were still dividing up the non-Trump vote.

In Iowa, Win It Back observed that in the areas where it ran ads, Mr. Trump’s likely share of the Republican vote fell by four percentage points. In the areas where the group did not advertise, Mr. Trump’s support grew by five points.

Mr. DeSantis has made his handling of the pandemic a centerpiece of his campaign. But the analysis suggests that this strategy leads to a dead end.

The memo says this of Win It Back’s most promising pandemic-themed ad: “This ad was our best creative on the pandemic and vaccines that we tested in focus group settings, but it still produced a backlash in our online randomized controlled experiment — improving President Trump’s ballot support by four points and net favorability by 11 points.”

Win It Back did not bother running ads focused on Mr. Trump as an instigator of political violence or as a threat to democracy. The group tested in a focus group and online panel an ad called “Risk,” narrated by former Representative Liz Cheney, that focused on Mr. Trump’s actions on Jan. 6, 2021. But the group found that the Cheney ad helped Mr. Trump with the Republican voters, according to Mr. McIntosh. (Link from article to ad: https://www.youtube.com/watch?v=3Mgjs7Gp874)

In a section of the memo titled “next steps,” Mr. McIntosh concludes, “We plan to continue developing and testing ads to deploy when there are signs of consolidation.”