I don’t think it’s cowardice, it’s more of a pragmatic view on the strong benefits of staying in lockstep as a party. Anybody doing politics in a national elected office is way past thinking that they can just vote their conscience on every issue. That may garner support from a few voters, but breaking ranks will cause the party to abandon a politician. And with the party apparatus and funding, there’s basically no way to ever hold ever national office again. I’m not aware of any national elected officials who got where they were without party support.