Flake is almost surely right on that second count. As for the first? Ehhh, I wouldn't be so sure.
Trump is a singular and thoroughly unprecedented figure in Republican Party (and American political) history; that much is certainly true. But there's a very convincing argument — which I will hardly be the first to raise — that he's more a result of today's shifting Republican Party than the cause of it. And if you believe that, it's much more difficult to dismiss Trumpism as a temporary “fever.”
Trump has certainly taken the GOP in a wholly new direction on a few issues, especially trade. But the things that really define him and separate him from other Republicans — attacking basically any establishment politician, fighting culture wars that most Republicans steer clear of, shunning all forms of political correctness — have been in demand among the GOP base for the better part of the past decade or more.
Before Trump, there simply hadn't been many GOP candidates capable of delivering that message and overcoming both the GOP establishment in the primary and the Democrats in the general election. Trump got it done — and on the grandest scale possible.
When such candidates were able to raise money and get their message out, as I've argued, they've almost always been relatively successful. GOP incumbents and establishment candidates have fallen in recent years to primary challengers who previously lost multiple campaigns for lower office and were otherwise bad candidates. Think Richard Mourdock, Christine O'Donnell, Sharron Angle, Todd Akin, etc. These candidates just couldn't win general elections.
And even when incumbents haven't fallen in these primaries, they've often won by closer-than-expected margins to little-known primary opponents pretty much whenever their opponents had money.
The parallels are certainly imperfect. Trump, after all, isn't the arch-conservative that many of these primary challengers have been. In fact, he's much the opposite: A former abortion-rights Democrat with little adherence to doctrinaire conservatism and little consistency in his positions. While they demanded purity, he's as impure as they come — at least on a policy level.
But in tone, he's basically the same. Republican primary voters loved how these upstarts upset the apple cart by going places other Republicans wouldn't on conservative issues, and they love how Trump upset it by going places other Republicans wouldn't with his mouth. The details for many seem to be unimportant; and indeed, if you look at polling, many in the GOP don't strongly abide by Trump's policies or his conduct. The important thing is that they were a middle-finger extended high to the establishment — of both parties.
So maybe Trump is just delivering on something for which Republican voters have been clamoring for a long time. If this is indeed a fever, it's arguably been rising for the past eight years. And the idea that it's going to break soon is something no Republican should assume.