'One of the most disgraceful things': Donald Trump slams provocative Ted Cruz ad

Real-estate mogul Donald Trump pounced Sunday after Sen. Ted Cruz (R-Texas) faced some backlash over one of the mailers for the senator's presidential campaign.

"I think it's one of the most disgraceful things I have seen in politics," Trump said in an interview on MSNBC's "Hardball."

"I think it's one of the most horrible things that I have seen in politics," the Republican presidential front-runner added. "And I have seen pretty bad stuff just like you have."

Trump was responding to the small firestorm generated by one of Cruz's recent campaign mailers in Iowa. Polls have placed Cruz as Trump's top opponent in Monday's Iowa caucuses.

The mailer in question gave supposed grades to both the recipient and his or her neighbors based on their turnout in recent elections. The top of the mailer proclaimed: "VOTING VIOLATION":

It's not an entirely uncommon trick of political psychology for campaigns to exploit: According to at least one study, people were more likely to vote when informed about the public nature of their voting histories.

Another presidential candidate, Sen. Marco Rubio (R-Florida), also released a "report card" mailer aiming to use social pressure, though his was slightly softer than Cruz's and generated less backlash. (The New Yorker's Ryan Lizza suggested that Cruz's "voting violation" notice may have fabricated the actual grades assigned to its recipients.)

For his part, Cruz reportedly defended the mailer by saying, "I will apologize to no one for using every tool we can to encourage Iowa voters to come out and vote."

Watch Trump discuss the mailer below: