The Memo: Here are the three states that will decide control of the Senate

The crucial battles in the midterm elections are coming into sharp focus with less than four weeks to go.

Republicans are expected to prevail in their quest to retake the House majority. But the Senate is a different story.

Control of the upper chamber, currently split 50-50, is on a knife-edge. The fight will be decided by whoever can eke out victories in a handful of states.

Three states are more important than all others — not only because the races are close but because they give one party or the...