Sen. Ted Cruz (R-Texas). REUTERS/Jonathan Ernst Real-estate mogul Donald Trump's constant questioning of Sen. Ted Cruz's (R-Texas) eligibility to be president is finally provoking a response from Cruz.

Cruz has long strained to avoid directly criticize the Republican front-runner. But on Tuesday, Cruz mused that it's "interesting" that some left-wing legal scholars are providing validation to Trump's claim.

Trump says voters should be concerned that Cruz was born in Canada and thus might not meet the constitutional requirement that presidents are "natural-born" citizens.

Cruz suggested on Howie Carr's Tuesday radio show that those scholars were working on behalf of former Secretary of State Hillary Clinton, the Democratic front-runner.

"She and Donald know each other well," Cruz said of Clinton. "And I do think it's interesting that Hillary Clinton's key supporters are doing everything they can to echo Donald's attacks on me. We've seen in the past couple elections where the Democrats worked very hard to ensure that they face the Republican they most wanted to face in the general election."

In another potential Trump tweak, Cruz said the polls have shown that Clinton would defeat the billionaire businessman "by a pretty big margin." (Trump would likely contest this claim, as there are also some polls showing him ahead of Clinton in the hypothetical matchup.)

"And it may be driven by the fact that the polling right now shows Donald loses to Hillary — and loses by a pretty big margin. But I beat Hillary. And I think that's got the Hillary folks a little bit concerned. And so they're doing everything they can to amplify Donald's attacks," Cruz told Carr.

Cruz stressed, however, that he doesn't intend to get into the "mud" of the 2016 race.

Since Trump started embracing the issue of Cruz's birthplace last week, Cruz has repeatedly stated that the law is clear and his American mother means he was a US citizen at birth and, accordingly, a "natural-born" citizen under the Constitution. Many experts back Cruz's legal interpretation.

On Tuesday, Carr also asked Cruz about Trump bringing up the senator's Cuban heritage at campaign rallies. Even though Cruz identifies as Southern Baptist, Trump has asserted that "not too many evangelicals come out of Cuba," a heavily Catholic country.

Cruz said the line could be a sign that Trump is getting "nervous" about his surging campaign.

"Well, listen, anytime someone is attacking your faith, that starts to suggest they're getting really nervous about what's happening in the race," Cruz told Carr. "I would be happy to invite Donald to come with me to church anytime he would like. He would certainly be welcome there."

He added: "My focus is to not engage in the mudslinging."