noun

the most active, committed, or strict members of a group or movement.

the party still has a hard core of supporters that will always vote for them

diehard,staunch,dedicated,committed,steadfast,hard-line,dyed-in-the-wool,long-standing,extreme,entrenched,radical,intransigent,uncompromising,rigid,deep-dyed

adjective

highly committed in one’s support for or dedication to something.

hardcore gamers