'How civil it's been': A suddenly subdued Donald Trump shows up at the big GOP debate

The tone of the Republican debate discourse shifted drastically Thursday, as the candidates repeatedly declined to attack each other 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 had constantly derided Rubio as "Little Marco," then did something completely 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. We don't bid out, as example, the drug industry, the pharmaceutical industry," he said.

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 to 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.' And my opponents on this stage not only didn't do the same, they attacked me," Cruz said.

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 at the debate, 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.

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

At a press conference on Tuesday night celebrating two additional primary wins, Trump said he'd 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 this coming Tuesday.