1508 people is plenty large enough as long as the sample design is well constructed.
It yields nothing to oversample when the sampling itself is prone to bias that skews results.
The Market Research Foundation Study Sampling plans are held in high esteem by marketing professionals. They are a proven resource.
Yes, Independents vote kitchen table issues and not GOP or Democrat issues. They do not trust the GOP; they do not like the GOP. They will vote for Ted Cruz in majority because of illegal immigration, Obamacare, Foreign Policy, Taxes and most importantly because they see the GOP establishment is set against Ted Cruz which strengthens his appeal as an independent-like candidate.
I hope you are right, but most people in this country have no idea the GOPe is trying to kill off the Tea Party, Ted Cruz and Mike Lee...