Not All Candidates Are Running For President to Become President
There are lots and lots of reasons to run for President, and only one of those reasons is to actually become President. Think of it, for an eight or nine month period of time, you're activities are followed by the National media. SNL, hopefully, gets a comedian to parody you, and just you, for the entire election season. So, if you're a politician, and you want to be known on a national level, running for President is a fantastic idea. Sure, it's more work, and you will probably lose the race, but winning is beside the point the first time around.
The king of running for President for reasons other than being President is the one and only, Dennis Kucinich, who's campaigns in 2004 and 2008 were laughable until we all realized he was running to meet women. And, it worked. Other candidates run for President in hopes of getting on the short list for positions with the next Administration, or run to get their issues front and center. Still others are running to become a nationally known politician.
Now, this point is key because when a politician is running for President for reasons other than becoming President, he/she will still run for President even when it's pretty clear that he/she has no shot whatsoever. This race is interesting because most of the candidates, including the frontrunners in the GOP race, aren't running for President to become President (at least this go around), they almost all have other reasons to run. So, if they have enough money to keep campaigning, few have any incentive to drop out.
On the other side of the equation, there are candidates who run for President with the sole goal of becoming President. These are candidates who have reached a level of national prestige that they don't need to run for President for people to know who they are. In this race, that was Hillary Clinton, Jeb! Bush, Scott Walker, Rick Perry, and Joe Biden. As soon as it became clear that Walker and Perry weren't going to be the nominee, they dropped out. Biden never entered the race because he wasn't going to be the nominee with Clinton in the race.
Jeb! Bush is Toast
When Jeb first stepped into the race, he had all the advantages - he had a pretty extensive name id, a $100 million SuperPAC warchest, and the backing of the GOP establishment. Those advantages gave him frontrunner status, which means that before GOP primary voters considered anyone else, they spent at least moment looking at Jeb! So, the fact that Jeb! is currently polling in the single digits (between 6-7%) is really, really, really bad. Now, in some cases, such as Kerry's 2004 run, the former frontrunner can make a comeback after the insurgent candidates flameout (think Dean scream), but only if there isn't another alternative. And here, GOP voters have both Kasich and Rubio to fall back on when Carson and Trump flameout.
Oh, and one other thing - since Jeb! is a Bush, and a national figure, every floundering moment of his dying campaign will be covered instead of ignored (which is what happened to McCain in 2008 and Kerry in 2004). That, in turn, will create more and more narratives of failure, and well, it's just bad. Jeb! is dead. He just doesn't know it yet.
Trump and Carson Will Flameout
The two frontrunners on the GOP side, Trump and Carson, are interesting fellows. Both jumped into the race without ever holding any political job, and both are doing fairly well in the polls - each are polling around 20%, a trend which has held steady for quite a few polling cycles. Now, everyone knows that these two will flameout, but everyone seems to think its because of something crazy one of them will say that becomes their undoing. It won't be, but it doesn't matter.
What does matter is organization. Presidential campaigns are huge undertakings that collect data from voters, processes that data, and then reaches out to those voters across multiple states. Now, a good organization won't help you if you are a crap candidate (sorry Jeb!), but if you are within striking distance, a great organization will propel you to the White House, which is exactly what happened in the 2008 Democratic Primaries, where Obama beat Hillary mostly due to his campaign's superior organization (more on Hillary later). This kind of an organization takes months to set up properly, but is absolutely essential, particularly in caucus states like Iowa.
But neither Trump nor Carson have much of an organization. Carson has raised a lot of money, but most of what he makes is spent on raising more money. Trump doesn't really raise money, but he isn't setting up very many campaign offices either. Neither man has put himself into a position to translate popularity into votes, and that's why, ultimately, they will flame out.
Cruz Has Put Himself in an Interesting Position
Ted Cruz is an odious Senator from Texas, and is so obnoxious that his fellow GOP Senators openly express their disgust for him. With that said, he is a clever politician, and he's placed himself in an interesting position. Right now, the GOP race is made up of outsider/insurgent candidates (Trump, Carson, Fiorina), and insider/politician candidates (Rubio/Kasich/Bush). But the insurgents are polling over 40%, collectively, and have been doing so since July. So, the GOP electorate is pretty sick of the establishment guys, but at the same time, the outsiders don't have the requisite campaign organizations to translate popularity into votes.
But Ted Cruz, smartly, has been putting together his organization, and he is positioning himself as the fallback guy when Trump and Carson flameout. He may not be the nominee, but Cruz will be a player.
Clinton Has the Nomination in the Bag
Now, I know there are a lot of Bernie Sanders' supporters out there, but guys, he's not going to win. I don't say that because of any particular failing of Sanders, but rather, because Clinton has responded to his candidacy the way that a frontrunner should - she's moved to the left. Now, call that shift to the left whatever you want, but as a frontrunner she has one very big advantage - every Democratic voter will look at her as a candidate first, and then if they don't like what they see, they will look at other candidates.
Also, Clinton has shown both in this campaign and in 2008, a resiliency that most candidates lack. When it looked like all was lost in 2008 after Iowa, and she teared up on the campaign trail, her campaign rebounded. This time around, she sat before a Congressional committee for 11 hours of testimony, and the Republicans couldn't lay a glove on her.
What's more, her campaign, particularly her social media staff, has been good this time around. Really good. I don't know if she'll win the Presidency, but she won't lose the nomination. And really, that's been the case for quite some time. If anything, having Sanders in the race has helped her by forcing Clinton to reach out to progressives and liberals. This is a good thing going forward.