Republican leaders are growing alarmed by the ferocious ways the party's mainstream candidates for president are attacking one another, and they fear that time is running out for any of them to emerge as a credible alternative to Donald J. Trump or Senator Ted Cruz of Texas. Leaders of the Republican establishment, made up of elected officials, lobbyists and donors, are also sending a message to the mainstream candidates, such as former Gov. Jeb Bush of Florida, that they should withdraw from the race if they do not show strength soon. The members of the party establishment are growing impatient...