There are plenty of reasons for skepticism about Mr. Schumer’s bubbly prognosis. To claim the Senate majority, Democrats need to pick up four seats while holding tight to vulnerable ones on a map that is downright ugly for them. Still, this was more than boilerplate boosterism. (I realize it can be hard to tell.) Earlier in the day, Senate Democrats had welcomed one of their most sought-after recruits to the fight: Mary Peltola, a former House member from Alaska, who announced her run to unseat the Republican Dan Sullivan.