Republican presidential candidate Donald Trump defended his position on the Iraq war during an interview with CBS's Face the Nation on Sunday, Feb. 14 following CBS's GOP debate in Greenville, South Carolina on Feb. 13. (Reuters)

President George W. Bush holds a news conference at the White House on Jan. 12, 2009. (Chip Somodevilla/Getty Images)

Nothing Donald Trump says has really damaged Donald Trump -- not calling illegal Mexican immigrants "rapists" and "criminals," and not calling for a ban on all Muslim immigrants. So why should his decision to publicly excoriate a popular former Republican president be any different?

At Saturday night's Republican presidential debate and into Sunday morning, Trump made clear that he thinks the Iraq war was a disaster and even suggested that George W. Bush was responsible for the attacks of Sept. 11, 2001. In the past, Trump has suggested that Bush should've been impeached.

It was all pretty striking on the eve of the former president debuting on the campaign trail Monday with his brother Jeb Bush, a GOP hopeful, at an event in South Carolina. And on its surface, it would seem to be a bad idea for Trump.

Why? Here's how The Washington Post's Scott Clement put it on our live blog during Saturday's debate:

Donald Trump’s criticism of President George W. Bush’s decision to go to war with Iraq and performance on keeping the nation safe from terrorism is out of line with attitudes of most Republicans in polls during Bush’s presidency — and afterward.

A January 2007 Washington Post-ABC News poll found 83 percent of Republicans thought Bush had made the country “safer and more secure,” while 17 percent said he had not. Later that year a Post-ABC poll found 85 percent of Republicans approved of the way Bush handled the U.S. campaign against terrorism, while only 12 percent disapproved.

Republicans are less unanimous in support of Bush’s decision on the Iraq war, but a a Post-ABC poll last May found 54 percent of Republicans saying the Iraq war was worth fighting, while 41 percent said it was not.

If hitting George Bush on the Iraq war is at all dicey, then hitting him on 9/11 would seem even more so. This is something, after all, that Bush supporters will note happened less than eight months into his presidency. Reasonable people can disagree, but that has certainly been enough for the vast majority of Republicans to give Bush the benefit of the doubt. Basically no major Republican before Trump has gone down this road.

So it all seems very wrong-headed, on its surface, for Trump to go after George Bush in this way. But here's one thing to keep in mind: Approving of Bush's presidency and his decisions is a matter of degree. It's one thing to say, 'Yeah, I think he was a good president and he kept us safe'; it's another to vehemently believe that and strike back at anyone who suggests otherwise.

On that second count, the Bush Defense has considerably less support.

In a March 2015 Washington Post-ABC News poll, fewer than half of Republicans (43 percent) approved "strongly" of Bush's presidency, while about as many said they approved "somewhat."

A September 2012 CBS News poll, meanwhile, found that just one-quarter of Republicans thought Bush was "one of the greatest" (6 percent) or a "very good" president (18 percent). About three-quarters rated him simply "good" (43 percent) or "fair" or "poor" (31 percent).

All of these numbers clearly tilt positive, but none of them do so overwhelmingly. By contrast, 41 percent of Republicans say Ronald Reagan was "one of the greatest" presidents, and 28 percent said he was a "very good" president. That's 69 percent giving Reagan at least "very" strong marks, vs. 24 percent for Bush.

In other words, plenty of Republicans like George Bush just fine and probably will be happy to see him on the campaign trail on Monday alongside his brother.

The number of Republicans who will go to the mat to defend his presidency against Trump's attacks? Probably much smaller.

Scott Clement contributed to this post.