Donald Trump just won his third primary, taking Nevada with a commanding 46 percent of the Republican vote. If you take a look back at his campaign so far, you will see a long trail of wishful Washington thinking that was proven wrong and had to be tossed to the side of the road. This started even before his campaign begun, as all the pundits confidently predicted he wouldn't even run. This was followed by a string of pronouncements that "Trump is now toast," after each of his ever-more-outrageous statements from the campaign trail (starting with his campaign announcement, where he called Mexican immigrants "rapists" and "murderers"). Each one was supposed to kill his chances dead, dead, dead. Instead, Trump has laughed all the way to where he is now -- the frontrunner and presumptive nominee of the once-proud Republican Party. He's getting tougher and tougher to beat, and his Nevada showing blew away the most recent wishful thinking from the Washington elites -- that Trump "had a ceiling" of perhaps 35 percent, above which he would never go. Toss that one on the ever-growing heap of things that have been predicted about how Trump's campaign was going to falter, none of which have panned out.

Democrats, meanwhile, have been mostly sitting on the sidelines, trying to contain their glee. Indeed, the common (and snarky) thinking on the left is that "it couldn't have happened to a nicer political party." They watch the wreckage of the establishment Republicans with amusement, confident that their nominee will handily defeat Donald Trump in November. To think any differently is to spit in the face of conventional political wisdom, after all. Except for one big thing they haven't even started to come to grips with yet: Donald Trump has gotten to where he is by continually spitting in the face of conventional political wisdom. It's what he does, and he's very good at it.

Which brings us to our main point. If Donald Trump is so hard to beat in Republican primaries, is he really going to be all that easy for a Democrat to beat in November?

This might seem to be a radical notion. Of course Trump will be easily beaten, Democrats tell themselves -- without ever considering whether that too might be one of those predictions that simply isn't going to come true this year.

I'm not saying Democrats should be panicking. We've got a long way to go before the general election. But I do caution that both Hillary Clinton and Bernie Sanders should at least be thinking about how they're going to take Trump on, should they become the nominee. Because it might not be the cakewalk everyone is now assuming it will be.

Right now it is looking like Hillary Clinton will become the Democratic nominee. Bernie still has a chance, but his path has gotten longer and steeper after losing Nevada. Assuming for the sake of conversation that Hillary does become the nominee, how hard is it going to be for her to beat Trump? Hillary is seen as a stronger candidate for the general by many, but even if Bernie Sanders wins the nomination, he'd then have his own problems (perhaps slightly different ones than Hillary) coming up with a strategy to take on Trump.

Head-to-head polling shows Clinton ahead of Trump right now, but not by as much as you'd think. This early, such polling is almost meaningless, but it should inject a note of caution for those assuming Trump will be easy for Hillary to beat. If he's such a loser as a Republican nominee, he should be doing a lot worse in such polling, even this early.

There is already a great deal of speculation about what Trump as GOP nominee would mean to general election voters. Again, Democrats are getting complacent by assuming there will be plenty of Republican voters who simply will not be able to vote for Trump in November. Some Republicans are sane enough to realize this is more than just a reality show, Democrats tell themselves, so they'll either hold their noses and vote for Hillary or they'll just stay home.

Perhaps, but this ignores a few things. In the first place, while such crossover voting might happen, that road travels both directions. There are a lot of Democrats who might decide they can't vote for Hillary Clinton, and either hold their nose and vote for Trump, or stay home. Take a look at her ratings from the general public on questions of honesty and likeability to see how vulnerable Clinton is to such voter defection.

Also, if Clinton beats Sanders for the nomination, there are going to be a whole lot of disappointed Bernie supporters, some of whom may decide they cannot vote for the woman who beat their guy. Add to this the segment of Democratic voters who are just flat-out sexist and wouldn't vote for any woman (to deny these voters exist is to deny reality, no matter how big this segment of Democrats may or may not be). Finally, there will also be some voters who see Clinton's progressivism as nothing more than a label of convenience for her, who might be susceptible to Trump's version of populism instead.

Again, I make no predictions about how big any of these groups may be, but merely point out that while there may be a stream of disaffected Republicans crossing over to vote for Hillary, there may also be a wave moving in the opposite direction to balance it out.

Trump's appeal to working-class voters should also be cause for concern among Democrats. There are four states that Democrats are almost complacent about winning that could shift the entire election, all heavy with blue-collar voters. Three of them have sitting Republican governors (two of whom threw their hat in the GOP ring for president this election cycle). Pennsylvania, Ohio, Michigan, and Wisconsin could be the keys to a Trump victory, in fact. Their Electoral College votes (PA-20, OH-18, MI-16, WI-10) propelled Barack Obama to victory twice, but if he had lost those states in 2012 he would have wound up with only 268 Electoral College votes -- and Mitt Romney would now be running for re-election.

Trump has one other thing going for him that few are exploring the ramifications of, at least so far. He's bringing a lot of new voters into the process. A lot of Republicans are voting, and a lot of them are first-time voters voting for Trump. Republican turnout has so far exceeded Democratic turnout in the early primary states. Even with all the excitement Bernie Sanders is bringing to the race, fewer Democrats are voting than in 2008 (using 2012 figures is an unfair comparison, since Obama ran unopposed for re-election). Low Democratic turnout combined with high Republican turnout should be a worrisome sign for Democrats. Bernie Sanders is bringing in a lot of new voters to the Democratic side, but if he fails in his nomination bid, how many of them will return to the polls to vote for Hillary in November?

Maybe I'm being too alarmist. Voters will have time to make up their minds in the general election -- Republicans will have time to get used to the idea of voting for Trump, and Democrats will have time to rally around either Clinton or Sanders. To put this another way, perhaps the general election won't be so different than in other (more normal) years.

But watching Trump blow away expectation after expectation in the primaries should really make Democrats to think twice about how easy it'll be to beat him in the general election. So far, Republican after Republican has tried attacking Trump, only to see their poll numbers sink and Trump's rise. Ignoring him or sucking up to him hasn't done Ted Cruz all that much good, either. The establishment Republicans are now seemingly afraid to even attempt taking on Trump.

Of course, Hillary Clinton or Bernie Sanders will have no compunctions about attacking Trump, and from a direction he's not used to (at least, so far). Even if Trump becomes the nominee, it's still possible he will say something so outrageous (perhaps in a one-on-one debate with Hillary?) that he drives away the independent voters who usually provide the winning margin in presidential contests. It's certainly a possibility, and if it comes to pass it'll be cause for much amusement on the Democratic side. If Trump bombs in the general election race, Republicans won't have any "Plan B" to fall back on -- they'll be stuck with him.

But such a meltdown is what Republicans have been hoping for all along, and it simply has not happened yet. Meaning it might not happen at all. Likewise, the firm belief that beating Trump will be a cakewalk for either Hillary or Bernie is also starting to look a little naive. It's just another version of the most-recent bad prediction -- the one that just got thrown on the rubbish heap of wrong Trump predictions. "Trump has a ceiling -- he'll never go above it" is really what Democrats are telling themselves now, regarding the general election. It's comfortable to think so, but Democrats should also not rely on it as being inevitable. So far, everything Trump has done has defied such expectations, so it would be risky to just assume this one will come true in November.