It’s legal, and it already happens on sites like Parler or Truth Social.
A smarter question is how that impacts the desire of most people to be on the site. Twitter’s rules were focused on behavior which most people find offensive rather than political viewpoints, and that meant that lots of people wanted to be there. The sites which only allow certain viewpoints tend to be a lot less enjoyable, and usually end in infighting as the purists turn on each other. Unsurprisingly, it turns out to be a bad business move to limit your potential user base and exclude advertisers who aren’t exclusively targeting that same narrow demographic.
Twitter censors liberals and leftists all the time. They just don't raise aa big of a stink about it as conservatives. Part of it is working the refs and part is the large increase in racist and conspiracy theories among right wing people in recent times.
A smarter question is how that impacts the desire of most people to be on the site. Twitter’s rules were focused on behavior which most people find offensive rather than political viewpoints, and that meant that lots of people wanted to be there. The sites which only allow certain viewpoints tend to be a lot less enjoyable, and usually end in infighting as the purists turn on each other. Unsurprisingly, it turns out to be a bad business move to limit your potential user base and exclude advertisers who aren’t exclusively targeting that same narrow demographic.