Ted Cruz is sincere in his conservativism, is far more married to his ideology than to Trump. Many, perhaps most, Republican congressmen are conservatives first and Trumpies seco... um, last.
I wonder what leads you to think that Cruz, Rubio, Lee, Scott, et al are more interested in Trump's popularity than their own beliefs. It's a rather odd view of Republicans, frankly, to view them *all* as *totally* devoid of principle.
For my part, I certainly give many Democrats credit for being True Believers. President Obama a prime example. Feinstein, Murray, Kaine and many Democrats are deathly serious in their stated beliefs.
If you're curious which Republicans are most 'principled' (or ideological) here is a useful scorecard. It's not the gospel truth, of course, but it's useful.