During appearances on network television Feb. 28, Republican presidential candidate Donald Trump repeatedly declined to refuse the endorsement of David Duke, a former grand wizard of the Ku Klux Klan. While Marco Rubio and Ted Cruz both took aim at Trump. (The Washington Post)

As it's become more and more clear that Donald Trump is the odds-on favorite to be the Republican presidential nominee, there's been considerable speculation about what he could mean for the broader GOP, particularly as the party tries to hold its Senate majority and consolidate its House margins in the 2016 election.

The answer: Nothing good — and perhaps something very bad.

While Trump's hard-line immigration policy (send 'em back, build a wall, make Mexico pay for it, etc.) has caused most of the hand-wringing within establishment GOP circles, the real danger for the likes of Senate Majority Leader Mitch McConnell (R-Ky.) and House Speaker Paul D. Ryan (R-Wis.) is not in that single issue. It's in Trump's remarkable unpredictability and seeming willingness to say things for the sake of shock value, and then inexplicably stand behind them — in fiercely unapologetic ways.

Trump's performance on the Sunday talk shows is indicative of this tendency.

Early on Sunday morning, Trump retweeted a quote from Italian fascist Benito Mussolini. The Mussolini account that Trump retweeted was part of an elaborate attempt from Gawker to dupe him.

Asked about it by Chuck Todd, host of NBC's "Meet the Press," Trump responded:

Look, Mussolini was Mussolini. It's okay to — it's a very good quote, it's a very interesting quote, and I know it. I saw it. I saw what — and I know who said it. But what difference does it make whether it's Mussolini or somebody else? It's certainly a very interesting quote. 

But wait, there's more!

Trump also appeared on CNN's "State of the Union" on Sunday. Here's an exchange between Trump and host Jake Tapper when he asked the candidate about the fact that several white supremacist organizations have spoken favorably of Trump's candidacy:

Trump: "Well, just so you understand, I don't know anything about [former Ku Klux Klan head] David Duke. Okay? I don't know anything about what you're even talking about with white supremacy or white supremacists. So, I don't know. I don't know. Did he endorse me or what's going on, because, you know, I know nothing about David Duke. I know nothing about white supremacists. And so you're asking me a question that I'm supposed to be talking about people that I know nothing about. "

Tapper: "But I guess the question from the Anti-Defamation League is, even if you don't know about their endorsement, there are these groups and individuals endorsing you. Would you just say unequivocally you condemn them and you don't want their support?"

Trump: "Well, I have to look at the group. I mean, I don't know what group you're talking about. You wouldn't want me to condemn a group that I know nothing about. I would have to look. If you would send me a list of the groups, I will do research on them. And, certainly, I would disavow them if I thought there was something wrong."

Tapper: "The Ku Klux Klan?"

Trump: "But you may have groups in there that are totally fine, and it would be very unfair. So give me a list of the groups, and I will let you know."

Tapper: "Okay. I mean, I'm just talking about David Duke and the Ku Klux Klan here, but ..."

Trump: "I don't know any — honestly, I don't know David Duke. I don't believe I have ever met him. I'm pretty sure I didn't meet him. And I just don't know anything about him."

In one Sunday morning, you have the most-likely Republican presidential nominee refusing, repeatedly, to disavow the KKK and saying, "Mussolini was Mussolini."

Make no mistake: Neither of these comments will adversely affect Trump in the upcoming Super Tuesday primaries and caucuses. For his supporters — and, at this point, that's a lot of people — his willingness to completely spurn the political-correctness police is the very thing that draws them to him. And, his unwillingness to apologize when scolded by the news media or other Republican politicians for some of his inflammatory remarks make his backers love him all the more: He's edgy! He's anti-establishment! He tells it like it is!

It works for Trump. Obviously. He has gone from a comic figure to the presumptive Republican presidential nominee in eight months.

The problem for anyone not named Trump — like the eight or so vulnerable Republican senators up for reelection in swing states — is that his unpredictability and love of controversy makes it almost impossible to deal with him as a factor in those races.

If the extent of Trump's controversial views was only his stance on immigration, that could be relatively easily handled by other down-ballot Republicans. For example, they could say: "I don't agree with Mr. Trump on every issue — we differ on immigration, for instance — but he understands that people are fed up with politics as usual and want a change after eight destructive years of Barack Obama." Not bad, right?

But, if you have no idea what Trump is going to tweet, retweet or say from the podium in front of thousands of people and dozens of TV cameras on a daily basis, that's hugely problematic for any Republican trying to calculate how to deal with him in their own campaign.

Having to respond to your presidential nominee's unwillingness to condemn the KKK or his seeming praise for the views of a fascist dictator in a single day — and with no idea what might come the next day — is the worst sort of problem for any candidate to deal with.

That's why Trump represents such a remarkable danger to the broader Republican Party as its presidential nominee. It's not that he has controversial views. (He does.) It's that he is totally unpredictable and undisciplined, careening wildly off message on a minute-by-minute basis. It works — or, at least, has worked to this point  — for him. But it's a total nightmare for any Republican looking at a tough reelection race this fall.