1. Our Constitution does not bare political parties. Today's political parties are Private institutions, hence, we don't elect majority leaders or candidates.
2.SC Justices get life terms in order to protect them from fear of a "controversial" decision. With automatic life terms, they can make a decision as they deem correct (according to the US Constitution, of course) without having to worry that their decision will affect them in the future( the opposite of a politician who has to get elected again). However, SCJ can be removed from their position if they are convicted of inappropriate behavior(Checks & Balances).
3. Fed Senators form the upper-house in the Congress alongside with the Representative(Lower House). In order for Congress to work or successfully pass something down, they must work in conjunction with each other Senate & Representatives. One without the other can't get far.