The most important choice American voters face in November is whether to re-elect Donald Trump as president. The second-most important is whether to leave Republicans in control of the Senate, whose assent is required to pass federal laws and to confirm presidential nominees to federal courts and senior jobs. If Mr Trump wins another term, Republicans will almost certainly hold the Senate as well. But if he does not, a Republican-controlled Senate would serve as a strong check on Joe Biden’s administration. If the record of Mitch McConnell, the Republican majority leader in the Senate, is any guide, a hostile...