Democratic held US Senate seats up in 2024 that does not have a Republican candidate.

HI(Hirono-D)
ME(King-I) vs Costello-D
NM(Heinrich-D) vs 2 Independents.
VT(Sanders-I) vs a Democrat(Svitavski-D) and Independent(Ericson-I)

In CA the Republicans will not make it out of the primary which is open.
CA-Schiff-D or Lee-D
HI-Hirono-D
VT-Sanders-I
ME-King-I
MA-Warren-D
MD-Alsobrooks-D
RI-Whitehouse-D
CT-Murphy-D
NY-Gillibrand-D
DE-Blunt Rochester-D
MN-Klobuchar-D
NM-Heinrich-D
WA-Cantwell-D
NJ-Menendez-D
VA-Kaine-D
MI-Slotkin-D
PA-Casey-D
NV-Rosen-D
WI-Baldwin-D
AZ-Gallego-D
MT-Tester-D
OH-Brown-D