Incumbent party candidate getting less than half their predecessor's vote

[Senator Incitatus]

In light of Shawn Wilson getting either 44.0% (primary) or 35.6% (general) of the Edwards vote, I thought I'd sniff around to find candidates from an incumbent party who got a vote total less than 50% of the incumbent Governor in the prior election, i.e., a total collapse.

I know this isn't a perfect comparison given Louisiana's system, but Wilson was essentially the Democratic nominee, and I wanted to point out that it is extremely rare in other states. (Even Dan Cox got slightly more than half of the Hogan vote. Even Cruz Bustamante got more than half of Davis voters in the 2003 recall.)

Incumbent party losses are often not a case of the incumbent vote cratering but by the opposition really gaining. Terry McAuliffe actually got more votes than Northam in 2021, and Bevin gained votes in 2019 despite his (alleged) unpopularity.

Here races I found since 1990.
In more than half of these cases, the presence of a third party in one or both races accounts for the decline. Edward DiPrete is the only one who was actually the incumbent himself, and that's basically because of two-year elections in Rhode Island:

Candidate	State	Year	Percentage	Prior Vote	Vote Total
Susan Collins	ME	1994	48.4%	243,766	117,990
Geoff Diehl	MA	2022	48.2%	1,781,341	859,343
Tim Penny	MN	2002	47.1%	773,713	364,534
Eunice Groark	CT	1994	47.0%	460,576	216,585
Edward DiPrete	RI	1990	45.3%	203,550	92,177
Mike McWherter	TN	2010	42.5%	1,247,491	529,851
Bruce Morrison	CT	1990	41.1%	575,638	236,641
Leslie Petersen	WY	2010	31.9%	135,516	43,240

Feel free to point out any I missed or go back further. I may return to expand the chart. (Fun Fact: In Mississippi, 1951, Democratic nominee Hugh White only got 26% of the vote that Fielding Wright received in 1947—and won the election. That was because Wright's election coincided with a five-way special election for the U.S. Senate. Otherwise, general elections in the solid South usually had abysmal turnout.)

[Unbeatable Titan Susan Collins]

Maine 1994 is just such a weird election, possibly benefiting of its own post at some point. The winner, King, won with less than 36% of the vote. Collins would two years latter more than double her vote share and become Senator defeating former governor and 1994 runner up Brennan, despte 1996 not being as good for the GOP as 1994. Collins and King of course now serve in the Senate together and have had a fairly close relationship.

Additionally at the same time the GOP saw its vote share halved in the Gov race they picked up the Senate seat with  the first lady of Maine running in an open seat.

It had most of Main's prominent politicians of since 1970 involved in some way.