Less than three months before the kickoff Iowa caucuses, there is growing anxiety bordering on panic among Republican elites about the dominance and durability of Donald Trump and Ben Carson and widespread bewilderment over how to defeat them. Party leaders and donors fear that nominating either man would have negative ramifications for the Republican ticket up and down the ballot, virtually ensuring a Hillary Rodham Clinton presidency and increasing the odds that the Senate falls into Democratic hands. The party establishment is paralyzed.