He isn’t leading in polls against Democrats. He’s leading in polls against 6 and 7 other Republicans. That’s a lot different.
Your wrong:
http://thehill.com/blogs/ballot-box/presidential-races/269702-hillary-trails-trump-head-to-head-poll
This shows the junior senator from the Lone Star state losing to Bernie
Sit at home, vote 3rd party, vote for the Democrat?
The reality is that Ted Cruz does even worse against either Democrat potential. If Trump wins the nomination then he most likely will get many of those votes now being spread across the other Republican candidates vying to win the nomination.
I for one am most interested in hearing your argument to support that assertion.