As midterm election campaigns heat up in the Senate’s top battlegrounds, the National Republican Senatorial Committee is canceling millions of dollars of ad spending, sending GOP campaigns and operatives into a panic and upending the committee’s initial spending plan. The cuts — totaling roughly $13.5 million since Aug. 1 — come as the Republicans’ Senate campaign committee is being forced to “stretch every dollar we can,” said a person familiar with the NRSC’s deliberations. Republican nominees in critical states like Ohio, Pennsylvania and North Carolina — places the GOP must defend this fall — have failed to raise enough money...