Senate Majority Leader Mitch McConnell promised the country Wednesday that a new justice will be confirmed to the Supreme Court this fall, setting out an ambitious goal for himself to approve a key seat with the narrowest of majorities. Democrats were already sounding obstructionist notes, however. Minority Leader Charles E. Schumer, New York Democrat, said the Senate should wait until after this year’s election, giving voters a chance to weigh in on some of the senators who will confirm the next pick.