I could see someone like Susan Collins endorse Hillary Clinton and announce she's becoming a Democrat.
Susan Collins is a lot more conservative than people seem to think she is. I can only assume it's because they're confusing her with her former fellow Senator from Maine, Olympia Snowe.
There are a number of Republicans in the Senate who are moderates in some areas like Bob Corker and especially Lindsey Graham, and maybe Susan Collins is one of those, but the only Republican left in the U.S Senate who I would consider a genuine across the board moderate is Lisa Murkowski.
My guess on Susan Collins is that she's actually more like Kelly Ayotte or Joe Lieberman used to be. Both of them are/were actually in the mainstream of their party on most issues but exploited a handful of high profile but most symbolic areas of difference.
In Ayotte's case, it's because she represents the Democratic leaning state of New Hampshire and knows that her high profile splits on a handful of high profile but meaningless issues with her party will overshadow her actual very conservative voting record with most journalists and voters.
I don't believe in engaging in 'pop psychology' so I wouldn't begin to hazard a guess at why, despite his mostly liberal voting record, Lieberman frequently went out of his way to praise Republicans and criticize his fellow Democrats. The issue that for a long time he trumpeted his conservative positions on was his dislike of 'Hollywood filth.' Since neither he nor Congress nor the President could actually do anything about that, it was a very easy issue for him to demagogue and play to conservatives with.
Of course, all of that was before September 11, 2001.
In answer to your unasked question though about moderate Republican voters, I would certainly expect that many in the managerial, professional and executive class who have voted for Republican Presidential candidates up to now would be more likely to vote for Hillary Rodham Clinton than for Donald Trump.
Even though Trump is basically one of them (though so is HRC) I'm sure most of them would find Trump to be far too erratic for them to support, given that business types tend to favor stability above everything else.
