The GOP was polling very well (or at least competitive) in CO before they nominated Trump. I'm not buying that CO is gone for Republicans.
More like it was never polled, except for this
Quinnipiac poll, that showed Rubio, Carson, Cruz, and Trump all beating Clinton by double digits. Yeah, complete and total junk.
CO Republicans have had the chair pulled out from underneath them on many occasions, and it all started with Obama's win in 2008. The 2014 Senate race was a perfect storm for them; bad year for Democrats, extremely low turnout, Udall was a terrible campaigner, and Gardner was probably the best candidate out there. All that and Gardner still only won by 1.9%.
Republicans can win without both states in the future if they nominate the right candidate.
That means they'd have to nominate Kasich or... Sandoval? The "right" candidate would never make it out of a Republican primary plain and simple.