“The Trump White House is clearly nervous about a serious primary challenge, as they should be.”
>yawn<
The author is saying that previous presidents who got challenged within their own party lost in November.
In Bush’s case, he got a primary challenge and then a third-party candidate in the general election.
But Weld reminds me more of Paul McCloskey, who ran against Nixon from the left (prompting John Ashbrook to run against him from the right.)