Senior US Republicans are searching for a "third-party candidate" to run in November's general election as part of a final effort to prevent either Donald Trump or Hillary Clinton becoming the next president. Both candidates are anathema to a swathe of conservative officials, who say Mitt Romney, their 2012 nomination, heads their list.High-profile Republicans - from former president George Bush to Paul Ryan, the party's most senior elected official - refuse to endorse Trump. Clinton is assembling a "Republicans for Hillary" group as she exploits disarray caused by Trump's ascent.Third-party candidates have always faced an uphill struggle but an independent candidate could attract enough support to prevent either Trump or Clinton - who elicits almost as much hostility as support - winning the 270 electoral college votes they need to become president.That would leave the decision in the hands of a Republican-controlled Congress, something that has not happened since elections in the 19th century.The search for a candidate is led by Bill Kristol, editor of the Weekly Standard, an influential conservative magazine. He met Romney last week to gauge his support.Trump completed his hostile takeover of the Republican party last week when, after his convincing win in Indiana, rivals Ted Cruz and John Kasich exited the race.That leaves a string of senior Republicans wondering whether they can vote for - much less endorse - a candidate who has alienated millions of potential voters with his attacks on Mexican immigrants, Muslims and women.At best, they fear he could hand the White House to Clinton. At worst, they worry the US would become a laughing stock if he were to win...