It's an important primary, because once a Democrat gets into the Senate from Hawaii, they're likely to stay there for a good long time. They're unlikely to get beaten by Republicans in a strong Democratic state. There's the possibility of a primary challenge if they do something really annoying, but who knows if that will work out. So the consequences of this small-state Senate primary could stick with us for decades, in whether we have an excellent Senator or an infuriating one.
It's important that other progressive candidates stay out of the race and not give Case the victory by splitting the progressive vote. If you're Colleen Hanabusa, Hawaii's other Democratic representative (and a fine progressive) your best chance of becoming a Senator is to endorse Hirono and not run this time. If you jump into the race, take votes from Hirono, and throw the election to Case, Hirono is probably going to be around to run against you for the open seat when Daniel Inouye retires. But if you endorse her and she wins, she'll be out of your way. And assuming that the favor-trading economy works properly, she'll help you in the election to replace Inouye. Maybe you'll have to deal with Case or some similar moderate, but it's a Democratic primary in a liberal state and you can take him in a one-on-one fight.