The graph tells the tale. One by one the anti-Trump candidates rise, helped in no small part by a desperate media and establishment, and one by one they fall. On December 20th Senator Cruz hti his highest level of 20.8. he has been in a steady decline subsequently and today is at 18.8
Cruz best poll was 24% it is now 17%
The Fiorina, Carson, Walker, Kasich, Bush, Rubio "surges" are noticeable, but like shooting stars are very brief. The exception being Ben Carson who, alone after Trump took the lead on July 20th, actually put his nose in front for three days. But in some sort of cosmic schadenfreude Carson's decline has been spectacular.
After Trump rose Bush declined. In Bush's stead Walker rose. After Walker declined Carson rose spectacularly whilst Fiorina, with the media, especially Fox's Megyn Kelly giving her every advantage, rose seemingly strikingly but from 1% which made her rise look bigger than it was.
Fiorina's decline saw Rubio's rise from a low of 5% to touching 15% and, as with all the other candidates, it has been steadily downhill from there to 11.5% These percentages are from the Real Clear Politics aggregate "Poll of Polls" so they smooth out the distortions possibly inherent in individual polls and seem the best method we currently have to gauge popular support.
None of the other candidates have risen above the seemingly impenetrable 5% barrier. Nothing could be clearer than the fact that Carson's decline, and to possibly a lesser extent Rubio's, has seen Cruz gain a polling rise almost equal to Carson's decline. This also clearly obvious in Iowa where Carson's Evangelical support has switched to Cruz.
Carson's polling height was 24.8 on November 4th on which day Cruz was on 8.8%.Today Carson is on 10% and Cruz 18.1%. But it is Cruz's current trend which is the key to whether or not Trump runs away with the nomination by "running the board" as his team advised they look to.
A few day polling is by no means a certain indicator of future trends, but with Trump's rivals it can be safely said that once a downturn trend commences it continues on apace. The Real Clear Politics graph above shows a slight leveling off for Cruz (the black line) which reflect the 6 point drop in Cruz's polling in the last two polls;
In the Reuters tracking poll polling Cruz's decline is more marked going from 17% on December 27th to 14.4% on December 29th
And in the Iowa caucus polling Cruz drops a huge 9 points.
Newly noted excellent analysis of North Carolina;
What looks like a Cruz "surge" in Nevada holds an interesting phenomena. In the last poll from Nevada from CNN Carson was on 22% in second and Cruz was on 4% (26% combined). In this Gravis poll Carson has collapsed to 6% and Cruz has"surged to 20%. But, as is obvious, they are on 26% combined having swapped supporters. Trump is still way out in front.
These moves may be temporary or may mark a plateau for Cruz who, on current polling stands a good chance in the Iowa caucus voting. But if these polls do, as with all the preceding candidates, mark a steady decline in Cruz's support then the road is open for Trump to simply destroy the opposition from Iowa right through to Florida (links to outstanding analysis from "Conservative Treehouse") and to the nomination.