Control of the Senate is a razor-close proposition in Tuesday’s election, as Republicans fight to retain their majority against a surge of Democratic candidates confronting the president’s allies across a vast political map. From New England to the Deep South, the Midwest to the Mountain West, Republican senators are defending seats in states once considered long shots for Democrats. Stunning amounts of cash have been flowing to Democrats from millions of Americans apparently voting with their pocketbooks; Republicans are tapping deep-pocketed donors to shore up GOP senators.