@evan I tend to think the biggest structural problem is gerrymandering. Not only does it distort the overall vote results, but it leads to electoral districts that always vote R (or D), and thus candidates who act extreme to appeal to the base rather than centrist to appeal to the swing voter.
A constitutional amendment could do something about gerrymandering, but there are plenty of non-constitutional measures too.
(I have a long list of ways I'd change the US Constitution had I the power.)