A rookie Senator facing his first vote reevaluates his platform and priorities when his wife is murdered in a mass shooting.

Richiev beat me to it.

I was going to suggest you make it so his party is for guns, but after his wife dies he is against guns.
So which party would best fit the bill (pardon the pun) and clarify that if he wants gun control he will be working against his own party?

I think the term 'rookie Senator' works if he is going against his own party.

In practical terms, (This is just me speculating out loud) He is a first term Republican senator who has always believed in the 2nd Amendment and is considered an up-and-comer within the party.

When his wife is killed in a mass shooting he reevaluates his position on gun control. However doing so endangers his career within his own party.

(It would help if he is a swing vote on a controversial gun control bill, and the passing or failing of the bill depends on his vote.)

This sounds interesting about exploring not only the challenges politicians face from exterior forces, but now you turned it about inner motives. I assume you are tackling gun control head on with this?

I don't much care for how rookie senator sounds. Perhaps young senator sounds better?