Can Donald Trump be stopped? The Republican front-runner's rivals, as well as the establishment wing of the GOP, have certainly been trying, but some strategies have been more effective than others.

Trump's remaining opponents in the Republican primaries, Texas Sen. Ted Cruz and Ohio Gov. John Kasich, as well as the super PACs that support them, have devoted a lot of resources to generating attack ads to halt his rise. Ace Metrix, a firm that measures the effectiveness of video advertising, has assessed how well those ads have played with both Republican and independent voters, and determined the most effective anti-Trump television ads to date. 

Independents appeared more swayed by ads in support of John Kasich, perhaps not surprising considering no GOP candidate polls better among independent voters. Those voters also appear to be more affected by ads that attack Trump as a fraud and a dishonest businessman. Among the most effective ads with independents is a series of videos by super PAC American Future Fund that features testimonials by ordinary Americans who claim to have been ripped off by the controversial Trump University.

The most effective ad with independents, "Talk Talk Talk," by the New Day For America super PAC supporting Kasich, argues that Trump is making empty promises to voters, while only Kasich has a proven record of job creation. 

Republican voters, by contrast, appear to be more swayed by ads that attack Trump for not being a true conservative, criticizing him for his positions in favor of Planned Parenthood and previous pro-choice statements on abortion. Among the voters surveyed, ads in support of Cruz appear to resonate more.

The two highest-rated ads among Republicans are "For Our Jobs," and "Talk Talk Talk," which both heavily feature messages about jobs and the economy, suggesting that effective economic policy may provide a route toward scaling back Trump's appeal ("Talk Talk Talk" is the most effective ad among independent voters surveyed as well). 

Here are the most damaging ads against Trump, according to independent voters surveyed by Ace Metrix, in order of effectiveness:

  1. "Talk Talk Talk" — an ad by the New Day For America that claims that while Trump is all empty rhetoric, Kasich has a proven record of creating jobs. 
  2. "Bob" — an ad by American Future Fund that highlights the story of a man claiming to be a victim of fraud from Trump University, Trump's academic venture.
  3. "Big Money" — an ad by anti-Trump super PAC Our Principles that lists allegations of fraud against Trump throughout the businessman's career. 
  4. "Kevin" — an ad by the American Future Fund that highlights the story of a man claiming to be a victim of fraud from Trump University.
  5. "Sherri" — an ad by American Future Fund that highlights the story of a woman claiming to be a victim of fraud from Trump University.

Here are the most effective anti-Trump ads, according to Republican voters surveyed:

  1. "For Our Jobs" — an ad by Cruz for President that criticizes Trump for his position on Planned Parenthood and claims Cruz can better deliver jobs for Americans.
  2. "Talk Talk Talk" — an ad by the New Day For America that claims that while Trump is all empty rhetoric, Kasich has a proven record of creating jobs. 
  3. "Plain Wrong" — an ad by Cruz for President that attacks Trump for supporting Planned Parenthood.
  4. "Nothing Conservative" — an ad by super PAC Club for Growth that lists examples of Trump previously supporting liberal positions. 
  5. "Supreme Trust" — an ad by Cruz for President that claims Trump can not be trusted to appoint conservative justices to the Supreme Court. 

As it stands, Trump has a commanding lead in the delegate count, with 678. Cruz is a distant second with 413 delegates, followed by Kasich with 143. Trump is also leading all candidates in national polling, with about 40 percent of the vote.