Republicans in US may back a third runner
If a third party candidate such as Mitt Romney runs in the United States Presidential election and neither Donald Trump nor Hillary Clinton get a majority of electoral college votes, Speaker of the House Paul Ryan will decide who will be President.
Establishment Republicans don't want Trump. They would support a third party candidate such as Romney and a vice-Presidential running mate who has the possibility of delivering a large number of electoral votes that would not allow any of the candidates to achieve a majority.
The election will be decided by members of Congress, who will vote along party lines with Ryan, a Republican and former running mate of Romney. Ryan will be directing them how to vote. As Kurt Vonnegut would say: "And so it goes."