Iowa narrows the US presidential field

Iowa doesn't pick nominees. It eliminates candidates.
Sometimes that happens quickly: On Monday night, Democrat Martin O'Malley and Republican Mike Huckabee announced immediately that they were out.
Sometimes it happens more slowly. Iowa Republicans just sent a strong signal to New Hampshire and national Republicans that only two candidates remain viable: Ted Cruz, who finished first with 28 per cent of the vote, and Marco Rubio, who as of this writing is in third place with 23 per cent, just barely behind Donald Trump.
Is it an absolutely definitive signal? No; in politics unlikely things can always happen. But no candidate has been a hopeless also-ran in the Iowa caucuses and rebounded to come anywhere close to a presidential nomination, and there's no reason to expect Rand Paul, John Kasich, Jeb Bush, Carly Fiorina, Chris Christie or Rick Santorum to seriously contest the nomination after this point. Santorum at least will probably drop out this week. Some of the rest may join him, wait until after New Hampshire, or even stay in longer, but their realistic chances are over. The same with Ben Carson, who did somewhat better but was never really viable to begin with.
Donald Trump? We don't know whether his polling numbers elsewhere will fall off as the campaign goes to other states, nor do we know whether losing in one place will destroy his message of being a winner. Both seem very possible, but we'll have to wait and see. If not, however, his overall unpopularity among Republicans still severely limits his chances as the field narrows further. Certainly, he's going to need more than a quarter of the vote (what he received tonight). Put it this way: To win, Trump needs this to be an "everything is different this time" election, and tonight was strong evidence that the normal political rules apply.
We'll see now how New Hampshire Republicans react to the signals coming from Iowa. Will moderate and mainstream conservatives jump on the Rubio bandwagon? Will Ted Cruz pick up serious support? It certainly wouldn't be surprising if support for Bush, Kasich and Christie's dips. Either way, national Republicans will almost certainly react to the signal from Iowa by pouring resources into the Rubio campaign.
One more thing on the Republican side. If almost no Republican party actors want Trump to win, and a majority also don't want Cruz to win, will they put pressure on Bush and other also-rans to stop attacking Rubio?
Put it all together and one can say: Yes, Iowa only eliminates candidates, but if we're down to only two with a plausible chance at the nomination, well, that's a lot of eliminating.
There's less to say on the Democratic side. It appears as I write this that Hillary Clinton will squeeze out a very narrow victory over Bernie Sanders. Nothing much changes. The Sanders showing was impressive, and there's no reason to expect his big lead in New Hampshire to dissipate, but there's still little reason to expect him to win the nomination.
All that said: There's a long way to go -- 49 states, plus Washington D.C., Puerto Rico, and more! Again, something unexpected can happen on either side. But tonight was a step away from "this time it's different" on both sides. And that suggests -- but certainly doesn't prove -- that the rest of the race will be predictable as well. - Bloomberg View