Down to the wire

i) The Iowa primary will be a test of Cruz and Trump alike. Iowa is the kind of state where a social conservative like Cruz should do well. If he can't win in Iowa, where can he win? Moreover, if Trump wins, that gives him momentum. That weakens Cruz in other states where a conservative candidate would normally have the advantage.

However, Iowa will be a test for Trump, too. Thus far, all we have is polling data. Iowa will be the first test of how many Trump supporters are Trump voters.

It will also expose how much (or little) he's invested in a ground game? Does he have a good voter turnout machine? 

Conversely, if Cruz wins, that will weaken Trump and strengthen Cruz for NH. 

If Cruz loses, that theoretically gives rivals like Rubio an opening. But they have to catch up and overtake Trump. 

ii) In terms of pure math, having twice the percentage points of your nearest rival is not enough to win the nomination. 20% doesn't get you the nomination. 

However, the primaries function as an elimination race. It's a linear process that quickly narrows. If Trump can pick up a string of wins at the outset, then other candidates drop out, at which point voters down the line no longer have a viable alternative. By process of elimination, Trump could win be default. He may raise the percentages if later voters have no one left to turn to. Indeed, that's how primaries work. 

iii) Suppose no candidate has enough delegates to win the nomination outright. If it's a brokered convention, then all bets are off.

iv) Suppose Trump wins the nomination. Can he will the general election? 

One advantage he has over Hillary (or Bernie) is that he's better in front of a camera. That, however, will be offset by biased debate moderators who favor the Democrat nominee. 

Theoretically, Trump might appeal to voters who don't normally vote for Republicans. But that's offset by all the Republicans who will sit out the election if he's the nominee. Moreover, his crossover appeal is dubious:

Contra Rupert Murdoch’s assertion about Trump having crossover appeal, Trump is extraordinarily unpopular with independent voters and Democrats. Gallup polling conducted over the past six weeks found Trump with a -27-percentage-point net favorability rating among independent voters, and a -70-point net rating among Democrats; both marks are easily the worst in the GOP field. 

http://fivethirtyeight.com/features/donald-trump-is-really-unpopular-with-general-election-voters/

v) If it's Trump v. Hillary, both candidates have astronomically high negatives, so they might cancel each other out on that score. Likewise, both have mountains of dirt to dump on their rival. In theory, that's a wash. But in practice, the mainstream media will target Trump. 

Typically, that would be offset to some degree by rightwing media. However, Trump won't get the same support from rightwing media that a conservative candidate would. 

vi) Then there's the question of how he'd finance a presidential campaign. Thus far I don't think Trump has had to spend much money because it's so easy for him to get free airtime. And the liberal media cooperates, both because it's good for ratings, and because they like to promote a civil war in the GOP primaries.

If, however, he gets the nomination, then the media landscape will drastically change. The same media that enables him as contender for the Republican nomination will turn on him as a contender for the presidency. He has two or three options:

a) Public financing

If he goes that route, he will be vastly outspent by Bernie or Hillary.

b) Self-financing

That would be a test of his commitment and sincerity. Is he prepared to gamble or squander his personal fortune on what might be a prospect with a dubious chance of success?

How much is he really worth? Isn't most of his personal fortune tied up in stock real estate? Assuming he has a controlling interest in The Trump Organization, surely he's not prepared to liquidate so much stock that he loses a controlling interest in his own company, is he?

Is he going to sell the Trump Tower? Trump Taj Mahal? 

Even if those would fetch enough money to finance his campaign, surely you can't sell that kind of real estate overnight. How would he liquidate enough assets, and liquidate them quickly enough, to finance his campaign out of pocket? 

c. Donations

How many working-class and middle-class Trump supporters will dig into their pockets to contribute to the campaign of a man who brags about his elite status as a multibillionaire? Won't they say to themselves, "If he's that rich, why doesn't he just write himself a check?"

Likewise, how many rich people will contribute to his campaign? Is he popular among the superrich? Or do they despise him as a braggadocio? 

Obviously, I don't have the answers to all these questions, and if he loses his nomination bid, they will be moot. But these are questions that prospective supporters should consider.

It would actually be in Trump's self-interest to drop out of the race before he's overtaken by events. Trump is a salesman who sells the product by selling himself. He's a brand-name unto himself. Thus far he's succeeded in raising his public profile for free. But if the campaign continues, and he suffers a humiliating defeat in November, or sooner, then his effort at self-promotion will backfire. Thus far in the process he had much to gain and little to lose. But that's about to change. A good poker player knows when to leave the table. If he was smart, he'd take his winnings and leave.