Beyond the local anxiety, a race that might normally pass under the radar nationally is instead drawing the attention of G.O.P. leaders as they try to maintain control of the Senate, an effort that has become increasingly challenging amid rising unemployment and faltering approval of President Trump ’s handling of the coronavirus crisis.With vulnerable incumbents in states like Colorado, Maine and Arizona, the prospect of having to devote time and money to retain a seat in deep-red Kansas is an unwelcome headache that infuriates many Republicans.