WASHINGTON — The nation's most powerful Republican told his party Monday he now is focusing on ensuring Hillary Clinton doesn't get a blank check as president with a Democratic-controlled Congress, suggesting he doesn't believe Donald Trump can win the election.

On Sunday night in St. Louis, Hillary Clinton and Donald Trump meet for their second of three televised 90-minute debates. This time, the debate will follow a town meeting format, with voters joining the moderator in asking questions. As NewsHour…