1. Trump declared on June 16. Scott Walker declared in July 13, nearly a month after Trump declared. Scott Walker was one of the last to declare, yet he was ahead in the polls for months with double-digit poll numbers running as high as in the 20’s, so you can't claim that Trump was polling 1 to 2 % earlier because he declared late. Nor can you claim it was lack of name recognition either. Everyone knows The Donald.
2. Take your mind back to this time in 2007 if you will. John McCain was polling at around 7% give or take and seemed to be stuck there. In fact pundits had started writing him off cause he couldn't seem to get his mojo going. Giuliani was polling far ahead of him. And it went just Giuliani either. Who ended up with the nomination in 2008? You tell me.
McCain, who announced in April of that year, was the immediate front-runner, then fell behind, largely because of the illegal immigration debate in the Senate (sound familiar?). He also ran short on cash in the late summer of 2007 (think Trump will have this problem?)
But Guliani had a number of damaging revelations already, most notably corruption with Bernie Kerick. So far, nothing has developed like that with Trump.
In fact, if anything McCain COPIED the Trump campaign by doing the talk show circuit and getting free press coverage for about two months until he reclaimed the lead in December 2008.
Guliani never, ever dominated every state the way Trump has. He led in one or two. The field also had two major jolts when Fred Thompson came in, then immediately crashed, and Mike Huckabee shot up in November.
If you want to compare Cruz to McCain (ideology aside) it's a huge stretch. McCain was early on the media darling, had been running his campaign for four years since Bush beat him, and had massive name recognition. So for him to go from 5-8 points into the lead wasn't nearly the challenge it is for Cruz.
Finally, Trump is 2:1 or 3:1 over nearest competitors. But the nearest competitor is never Cruz. He is 5:1 or 6:1 ahead of Cruz in most of these states. That's a far, far cry from where McCain was.