1. This has a real possibility to pass.
2. This puts the issue on the table and forces people to take a very public stand on the issue.
3. This issue is supported by some 80% of the population, but Dems can't support it without alienating their base.
4. The actual passage is not as important as increasing our lead in the Senate and retaining the Presidency, thereby getting a majority of judges who won't do this crap.
This isn't some wildly fanatic thing to do. Besides making moral sense, it is a calculated political step to out the Dems as way way way out of the mainstream.