Donald Trump with Ted Cruz at the CNN debate. Joe Raedle/Getty Images The tone of the Republican debate discourse shifted drastically Thursday night, as the candidates repeatedly declined to attack one another as harshly as they had previously.

Republican frontrunner Donald Trump quipped more than 30 minutes into the debate, "So far I cannot believe how civil it's been up here."

Trump in particular has been known for his relentless criticism of his foes, especially when they attack him first. But he shrugged off attack after attack on Thursday night.

During a discussion about Social Security benefits, Sen. Marco Rubio jabbed and said Trump's numbers didn't "add up."

The debate moderator invited Trump to respond.

Trump, who in the previous debate had derided Rubio as "Little Marco," then did something new: He largely ignored Rubio's attack and simply repeated his claim that there were entitlement savings to be found.

"I've been going over budgets and looking at budgets — we don't bid things out," he said. "We don't bid out, as example, the drug industry, the pharmaceutical industry."

Later, when ethanol mandates came up, Sen. Ted Cruz took a shot at Trump. Cruz suggested that the frontrunner had pandered to Iowa's corn industry. But the back-and-forth was anemic compared with that of past debates.

"When I went to Iowa and campaigned against ethanol mandates, everyone said that was political suicide: 'You can't take on ethanol in Iowa,'" Cruz said. "And my opponents on this stage not only didn't do the same, they attacked me."

Trump accused Cruz of changing his stance on ethanol, but his language was toned down significantly from past attacks. In almost every stump speech leading up to the debate, Trump had repeatedly referred to Cruz as "lyin' Ted."

But on Thursday night, Trump simply said:

If you look back to Iowa, Ted did change his view and his stance on ethanol quite a bit. He did — and at the end — not for long, but he did change his view in the hopes of maybe doing well. And I think everybody knows that, it was a front-page story all over the place, and he did make a change.

The rest of the debate mostly conformed to this theme, with the candidates taking weak shots at one another but keeping it relatively tame compared with their past fireworks.

After the debate, Trump praised the tone of the debate.

"I thought it was a very elegant debate, I thought it was very substantive … and I thought folks did a great job," he said on CNN. "We needed this kind of debate, we needed this kind of tone, and I'm glad it took place tonight," he added.

As Trump comes closer to clinching the Republican nomination for president, he has started sounding more like a general-election candidate.

At a news conference on Tuesday night celebrating two additional primary wins, Trump said he would help other Republican candidates, mentioned a conversation he'd had with House Speaker Paul Ryan, discouraged the crowd from booing Mitt Romney, and talked about beating Hillary Clinton in the general election.

The candidates are heading into another round of big primaries on Tuesday.