The vast majority of Congressional districts in US are non-competitive. This means that what matters in those is the primary, not the general election. So if you're a liberal in a solid red district, threatening your Republican elected representative probably won't do much good (unless it's a swing district - those guys actually listen to some extent, especially now that Dems won a bunch of special elections in the suburbs, and there's talk of a wave next year).

But if you're a liberal in a blue district, then you should write your rep and let them know that 1) you will vote in the primary, and 2) having a firm public position on this issue will be your litmus test for who gets your vote.