Republican front-runner Donald Trump on Monday seemed to agree that the presidential candidates shouldn't attack each others' wives or kids -- but he suggested his opponent Ted Cruz is the one who really needs to commit to that standard.

Asked by Wisconsin radio host Vicki McKenna whether kids and wives should be off limits, Trump said, "Well, that's okay, but all you have to do is tell that to Cruz. Because he started it."

Trump once again blamed Cruz for the feud involving their wives. It started with an anti-Trump ad last week, which used an image of Trump's wife Melania. She is posing nude in the photo, which was taken for GQ magazine, and the ad says, "Meet Melania Trump. Your next first lady." The ad was run by an anti-Trump super PAC unaffiliated with the Cruz campaign, but Trump on Monday again insisted that Cruz was behind it.

Trump told McKenna that it was "really naive" to think Cruz wasn't behind the ad. Reminded that a super PAC rand the ad, Trump responded, "a Super PAC that he is friendly with." Trump also again denied that he was behind a story that ran in the National Enquirer on Friday, claiming to reveal unsavory details about Cruz's personal life. "I don't know who did it, but it wasn't me," he said.

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Cruz, for his part, told reporters in Wisconsin Monday that the National Enquirer story was "complete garbage" and blamed Trump for it.

"It is total lies, it was planted by Donald Trump's henchmen, and I don't think the people of Wisconsin or the people of America have any interest in tabloid trash," he said.

Cruz also challenged Trump to a one-on-one debate, as an alternative to the two televised town halls scheduled for this week.

"In all likelihood, Donald is going to turn me down," he said. "Why? Because he's scared of actually answering questions about substance. Because while he's very good at attacking and yelling and insulting, he does not have meaningful solutions to the problems facing this country."

As the two candidates continued their personal attacks against each other on Monday, Trump also stepped up his challenge to the allocation of GOP delegates from Louisiana. To handle this and other delegate issues, the Trump campaign has hired veteran Republican strategist Paul J. Manafort, a senior Trump adviser confirmed to CBS News. Manafort helped manage the convention floor for President Gerald Ford in 1976, when Ronald Reagan challenged Ford's claim to the GOP nomination.

The chances for a contested convention could increase next week, if Trump doesn't win the Wisconsin primary on April 5. Trump heads to the state on Tuesday, but Cruz has already been making inroads there. Republican Gov. Scott Walker is expected to announce on Tuesday morning who he's supporting in the primary.

Asked whether he will be seen with Walker on Tuesday, Cruz wouldn't say, but he praised the governor and said he would welcome his support.

"This is a job interview," he said. "We are campaigning, we are doing a bus tour all across the state of Wisconsin. We are answering the hard questions, and we are building support one voter at a time."

A poll released last week by Emerson College showed Cruz leading Trump by just one point, 36 percent to 35 percent, in the Wisconsin primary. Ohio Gov. John Kasich received 19 percent support in the poll.