If you contact your congresscritter over their official congressional channels - email, phone, Twitter, plain ol' letter - and tell them you plan to vote for/ag

If you contact your congresscritter over their official congressional channels - email, phone, Twitter, plain ol' letter - and tell them you plan to vote for/against them because of $X, a very polite intern will tell you that they will record your support/opposition to $X but that they cannot pass along your comment on voting to your congresscritter. Be sure to share that with their campaign: that's how you get that message through.