GOP insiders say they see 3rd party challenge if Trump is nominated

[Beefalow and the Consumer]

Am I misreading this - how would that prevent the Democrat from winning if he/she had 270+ EVs?

It wouldn't.  The goal would be to prevent both Trump and the Democrat from getting to 270 by going all in on a few key states.

If Trump is the candidate, that throws a bunch states into the Democrats' column that it would be very hard for an independent Republican to take, even with a concentrated focus.  You need to appeal to:

Heartland states (Ohio, Indiana, Missouri)
Western swing states (Colorado, Nevada, Arizona)
The Nouveau Mid-Atlantic (Virginia, North Carolina)
and... oh, yeah: FLORIDA.

What candidate is going to do that?  Know of any Latino generals/admirals from Ohio?