Republican presidential hopeful Donald Trump is 250 delegates ahead of his next closest rival for the party’s nomination, so knocking him off his perch at this point requires a devastatingly effective approach. But the recent TV ads targeting the frontrunner are having a lukewarm effect, at best, an analysis of anti-Trump commercials by the ad-scoring firm Ace Metrix found.

Only about half of the anti-Trump commercials studied by Ace Metrix scored above the norm for political ads—which indicates that they’re effective at reaching voters—among both Republicans and independents surveyed.

Donald Trump’s rivals and super PACs have funneled more than $63 million into advertising to undermine the candidate’s lead. But still, the frontrunner continues to rack up delegates in the race for the nomination.

Ace Metrix scored the 24 anti-Trump ads released since late January, which made up 21% of all Republican political commercials during that time period. They interviewed 500 national registered voters per ad, and measured their responses using eight metrics—whether people would watch the ads again, thought they were persuasive, agreed with the overall messages, were driven to seek more information, learned something from them, thought they were credible, found them relevant, and whether the ads grabbed their attention.

Among voters who self-identified as Republicans, 13 of the 24 ads studied scored above the norm for political ads. Some of the most-effective, attention-grabbing, and credible ads, according to the study, focused on Trump’s inability to win over conservatives and attacked his stances on the issues. Meanwhile, ads that didn’t work with this group positioned Trump as a Democrat, which voters apparently didn’t believe, and slammed his values.

Most effective anti-Trump ads among Republicans

Least effective anti-Trump ads among Republicans

Title Sponsor Ace score below the norm “Playing Trump” Cruz for President -39 “The Best Words” American Future Fund -47 “New York Values” Cruz for President -48 “Democrat” Our Principles PAC -61 “System” Cruz for President -63

At this late stage, the anti-Trump ad blitz may not make much real difference in the primaries, Mark Bryant, vice president of Ace Metrix Politics, told Quartz.

And in any case, political advertising is playing a less important than usual role in this primary campaign. While Trump himself has one of the smallest advertising budgets, he dwarfs his rivals when it comes to what’s called “earned” or “free” media—news and commentary in newspapers, on TV, online, and in social media about his campaign, as the New York Times’ Upshot blog reported.

Furthermore, “the segment of the electorate that’s turning out for Trump—described as Reagan Democrats and non-college-educated angry white males—are really just not affected by anti-Trump ads,” Bryant said. “They’re going to vote for him no matter what.”

Overall voters, including those who identified as Democrats or independents, as well as Republicans, found the ads pretty mediocre as well:

Most effective anti-Trump ads among all voters

Title Sponsor Ace score above the norm “Bob” American Future Fund +125 “Quotes“ Our Principles PAC +116 “Talk Talk Talk” New Day for America +99 “Kevin” American Future Fund +92 “Sherri” American Future Fund +85

Least effective anti-Trump ads among all voters

Of course, a lot of the funding behind the current wave of anti-Trump ads is coming from Republicans trying to prevent him getting the nomination. If Trump ends up representing the party in the general election, it remains to be seen whether Democrats can do any better with their political attack ads.