WASHINGTON — The conservative Christian coalition that helped usher President Trump into power in 2016 is planning its largest midterm election mobilization ever, with volunteers fanning out from the church pews to the streets to register voters, raise money and persuade conservatives that they cannot afford to be complacent this year.

But the cumulative weight of scandals in Mr. Trump’s private and public life is threatening to overshadow what the religious right sees as its most successful string of policy victories in a generation. And Republicans will be up against not only a resurgent liberal opposition to Mr. Trump but also the historical disadvantages that burden any party in full control of Washington, especially in the first off-year congressional elections of a president’s term.

“The midterms are going to be very, very tough for the Republicans,” said Robert Jeffress, who leads the First Baptist Dallas megachurch and is one of Mr. Trump’s most loyal evangelical supporters.

The vast majority of evangelical Christians are digging in for Mr. Trump, despite accusations by a pornographic film star and a Playboy playmate that he had separate affairs with them shortly after his wife, Melania Trump, gave birth to their son. Those controversies, paired with the multiple women who accused him of groping them before the election and his own boasts of sexual aggressions, have highlighted the unyielding support of a political bloc that once put moral behavior at the center of its political judgment.