Senate Democrats offered their ante Thursday in the renewed gun control debate, saying any bill must expand background checks and allow court-ordered gun confiscations while demanding a vote on a semi-automatic rifle ban Senate Minority Leader Charles E. Schumer of New York said his Democratic caucus is prepared to provide a “very large” number of votes for those priorities. “This is what we think should be done. This is the right thing to do” Mr. Schumer said.