It gives workers and retirees different benefit options. Supporters say it will help get the system back on track. But, critics argue it doesn't do enough in the way of savings.
They also wonder whether it's constitutional. It's a question one senator says is hard for anyone to answer.
The bill now heads to the House for a vote. It's hard to say what kind of support it will get there. That chamber just passed a reform bill of its own.