They can only refuse to seat if he is not constitutionally qualified, by court precedent.
WHAT ARE YOU TALKING ABOUT?!?
This is the US Senate. Have you not read the Constitution? Article 1 Section 5 gives them the power to EXPEL any member they want upon a 2/3rds vote. The Democrats once EXPELLED 10 SOUTHERN SENATORS AT ONCE!
People need to get a clue. Stare decisis means NOTHING in a case like this.