BCO07 said:
I mean how trump is outperforming incumbent republican senators ie Cruz
I don't think the margin between Trump and Cruz will be that much; my guess is probably about 1.5%-2%.
That being said here are my thoughts as to why. Here's, IMO, the main reason.
Some of Trump's supporters are very new to the idea of voting Republican, and won't consider voting for a Republican that is not sufficiently differentiated from the Bush-era, Wall Street-Neocon dominated Republican Party. I think we will see this dynamic playout, specifically, with Trump running a few points better than Cruz with Hispanic, black, working class, and younger voters in Texas.
Off the top of my head here are a couple other lesser/possible explanations.
Cruz was already a reasonably well-known political commodity prior to the rise of Trump (he emerged as a Tea Party era favorite). If you are one of the small portion of Trump voters that in 2008 (or even 2016) was an Obama voter (because you thought - wrongly - that Obama was actually going to govern in opposition to the establishment), you're negative first impressions of Cruz from back then might still hold sway.
A very, very, very, very tiny fraction of Trump voters might still hold a grudge against Cruz from the 2016 campaign. This is silly of course, but I'm sure there are a few voters out there that just don't like/trust Cruz because of it.
Vance in '28