Because when a Republican (RINO in this case) does not present a large difference between themselves and a Democrat, the Democrat wins. Republicans don’t want any more RINOS.
If Collins loses its a good chance we lose the Senate and say hello to majority leader Schumer. And even if Trump wins he without the senate he would be a lame duck presdident. No more judges, Schumer would make sure they don’t even get out of committee
We need every senator, RINO or not right now as McSally is likely to lose in AZ
“Because when a Republican (RINO in this case) does not present a large difference between themselves and a Democrat”
I believe this was the fatal flaw behind Reagans “Big Tent” strategy. When you get a bunch of democrats enlisted to your cause you get what you ask for. “why did you bite me?” asked the foolish woman “after i cared and nurtured you back to health.”, “because i’m a snake” replied the snake “thats what we do!”. It became “the UNIPARTY” with people like McCain who were profiting from being ‘maverick’ and ‘independent’. In other words, reliable democrats.
We need Republicans, even RINO’s to maintain control of the Senate.
And now, she is acting like a democrat, talking like a democrat, voting like a democrat. So, her used-to-be-republican voters say, “Why not vote for a democrat?”
(And the always-a-democrat voters say “Let's vote for a democrat this time!” With the excitement noted.)
She has one chance - And her staff, and her foolish pride, and her foolish hatred of Trump, will not let her make it - Call Trump. Apologize, and ask for an endorsement and a few campaign visits.
Your analysis is a bit off - she normally gets close to 70% of the vote. She is being excoriated in this race for supporting tax reform and voting to confirm Kavanaugh. The ads are attacking her as a "Trump stooge", etc.