Democrats blocked a vote Tuesday on cracking down on sanctuary cities as the first day of the immigration debate began with a whimper. Sen. Charles E. Schumer said debating sanctuary cities would be getting off “on the wrong foot,” insisting this week’s floor fight should only be about illegal immigrant “Dreamers” and border security. The early jockeying underscored the problems the Senate is about to face. Democrats had begged for a “fair” process to debate immigration, but are now worried about just how wide-ranging that debate will be, and are trying to shape the amendments that will be offered.