If everything goes according to plan, the Senate will begin debate Thursday on the first major federal gun legislation to have a real chance in two decades. Before the announcement of Mr. Manchin's proposal, drafted with Sen. Patrick J. Toomey (R-Pa.), the prospects of 60 senators agreeing even to begin that debate were murky. With the compromise plan on the table, the chance that the country will finally get some up-or-down votes on basic gun measures looks better.