WASHINGTON - During a discussion about his party’s legislative high points this year with a small group at the U.S. Chamber of Commerce last week, Senator Mitch McConnell, the majority leader, expressed a new concern about an old habit of President Trump’s. The many “distractions” generated by the president, Mr. McConnell said during the dinner, were preventing Republicans from having a coherent message for the midterm elections focused on the booming economy, according to multiple people who were briefed on the remarks. Representative Paul D. Ryan, the House speaker, who also attended, expressed another concern - that the president’s talk with his supporters of a “red wave” in November was unfounded. All agreed that he should instead be sounding the alarm about the possibility of big Democratic gains. The two congressional leaders were only echoing the worries of many Republican strategists and Mr. Trump’s own advisers.