This is the end of the line for Trump

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Donald Trump is getting closer to clinching the GOP presidential primary nomination after big wins in Florida, North Carolina and Illinois on Tuesday, even though he lost in Ohio.

But that's where the good news for Mr. Trump ends, because he's about to find out that what's worked for him in the early stages of his campaign and through the primary elections won't carry over to a general election contest. And that's especially true with Hillary Clinton being the likely Democratic presidential nominee.

There are the three key reasons for the Trump campaign's impending November doom:

1) Primaries are like congressional midterms. They draw a much smaller number of voters with a much more defined agenda. Name recognition, the kind that Trump has enjoyed for years, is more crucial. The primary and midterm voters are also usually angrier than the average presidential general election voter, and that has worked in Trump's favor since last June.

It's impressive and unprecedented that Trump has been filling large stadiums and arenas during the primary stage of this election. But in general elections, stadium-sized crowds are standard fare. So while those Trump primary voters are sure to come out in November for the general election, they're going to have a lot of company that's likely to drown out a bigger portion of their voices. By definition, general elections are thus tipped more in favor of the more moderate candidates.

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2) Trump really can't track to the middle. Can you even keep a straight face when considering whether Trump could choose someone like Jeb Bush as his running mate? I didn't think so. Trump has painted himself into such a corner that trying to attract more mainstream voters by choosing an "establishment" running mate or significantly softening his tone seems impossible.

To be clear, there's nothing to physically stop Trump from doing those things. But moderating his image, message, or campaign team would surely alienate his key base of voters who not only want change, but do not want to truck with any insider figures. Trump is more likely to choose someone like Ben Carson or another non-Washington insider running mate and that means Trump will have to use his VP pick to play defense with his own base as opposed to going out on offense to grab more moderate voters.

Meanwhile, Hillary Clinton's major campaign message already seems to be shifting to portray itself as the sane alternative to Trump and the Trump rhetoric that gives so many voters pause.

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3) This is going to get uglier. I believe the nasty, violent, and clearly coordinated protests disrupting Trump rallies this weekend are actually helping his primary chances for now. At this stage, they make Trump look like the victim of decidedly liberal and non-Republican groups. But if there's similar chaos at the Republican Convention and at Trump rallies in the general election, the perception will go from sympathy to fear, scorn, and a legitimate worry that a Trump who can't keep the peace at his campaign rallies surely won't be able to keep America safe as president.

That's what we saw most famously for the Democrats in 1968, when their violent Chicago convention significantly weakened nominee Hubert Humphrey from the get-go. What a lot of people forget is that it didn't stop in Chicago as summer turned to fall and hecklers relentlessly kept dogging Humphrey's campaign appearances. That combination left him way behind in the polls. Humphrey and running mate Sen. Edmund Muskie finally turned the chaotic tide late in the game when they started letting some hecklers come on stage and speak briefly into the microphones, making them look like elder statesmen by comparison. But it was too late. The surge in the polls Humphrey enjoyed in late October wasn't enough to carry him over the finish line ahead of Richard Nixon, who won the popular vote by a hair.

I bring up the 1968 example not only because that election was nasty, but because it was a decidedly violent and divisive year in American history. It was also a year of unprecedented American casualties in the Vietnam War. 2016 is, thankfully, not as violent a year. But the campaign rhetoric is probably nastier this year and the electorate is just as angry and divided, maybe more so.

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I always laugh at "experts" who routinely use ill-advised words like "unprecedented" when describing negative politics in modern elections, because nothing can really beat the nastiness of the elections of 1800 or 1828 where sitting presidents were accused in the established newspapers no less of everything from being hermaphrodites to pimps.

But the physical violence that's ensued at Trump rallies really does look like it's going down an unprecedented path. I'm not sure the Trump campaign or the Secret Service can keep the peace at these events going forward. And that's the final major advantage for Hillary Clinton, who's made choreographed political events her bread and butter for decades now. And thanks to Trump's extremely high negative polling numbers, those choreographed Clinton rallies will have every incentive to be centered around the leitmotif of Trump as evil incarnate.

It will be a nasty race through and through. And when things get nasty, voters often go with someone they don't personally like but they see as the only reasonable option. That's exactly how Nixon won the White House in '68, to use one last example from that very similar and instructive year.

Trump's unexpected and meteoric rise to the GOP nomination is about to be secured after Tuesday's primaries. But this is as far as he goes. General elections, even in crazy years like 1968 and 2016, are a different kind of deal that even The Donald can't close.

Commentary by Jake Novak, the supervising producer of "Power Lunch" and former supervising producer of "The Kudlow Report." Prior to joining CNBC, Novak co-created and oversaw the "Varney and Company" program on FOX Business Network along with anchor Stuart Varney. He also spent seven years at CNN, producing financial news programs including launching the successful "In the Money" show with anchor Jack Cafferty.

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