Democrats See a Rare but Fleeting Chance to Shake Up the Senate Map

Democrats are targeting G.O.P. strongholds that could set them up for new majorities. But if they don’t win those races now, it could be a long time before they have another chance.Protesters outside of an event for Senator Roger Marshall in Topeka, Kan. Soaring fuel costs and economic struggles have made some Republican senators like Mr. Marshall vulnerable on their home turf.