THE ATTACK by Mitt Romney, the 2012 Republican presidential nominee, on Donald Trump, who looks likely to be the party's nominee in 2016, is not unprecedented. It has a parallel in the presidential election of 1912, when a split in the Republican party led the sitting president, William Howard Taft, to finish third in the polls. Taft ran foul of his predecessor, the energetic and volcanic Teddy Roosevelt, who ran on the Bull Moose ticket. The effect was to hand the presidency to Woodrow Wilson, the first Democrat to win office since 1892.
Roosevelt had regretted his rash vow not to run for a full third term in 1908 (he took office in 1901 after the assassination of William McKinley; this was before the passage of the 22nd amendment, which limited presidents to two full terms). While Taft was Roosevelt's hand-picked successor, and a long-term friend, the latter gradually became disillusioned with the Taft administration. Broadly speaking, Roosevelt was a populist, favouring higher tariffs and "trust-busting" attacks on big business; he was also a conservationist, setting up the national park system. Taft was more of a conventional, business-friendly conservative.
Taft's control of the Republican machine meant that he was able to deny Roosevelt the party nomination at the convention, amid disputes about the legitimacy of delegates. But Roosevelt was popular in the country and accepted the nomination of the reform-minded Progressive party. This was a fluid time in American politics; both William Jennings Bryan, who lost three times as a presidential candidate for the Democrats between 1896 and 1908, and Woodrow Wilson, had progressive leanings. The Republican party had its own progressive wing, featuring not just Roosevelt but Robert La Follette too. There was even a Socialist candidate, Eugene Debs, who picked up 6% of the vote.
Roosevelt performed better than any subsequent third party candidate has managed, finishing second, picking up 27.4% of the poll and 88 votes in the electoral college. (For comparison, Ross Perot got 18.9% but no electors in 1992; George Wallace claimed 45 electoral votes and 13.5% of the poll in 1968.) The long-term result of the 1912 election was to force progressives out of the Republicans and into the Democrats, eventually leading to the coalition led by Teddy's distant cousin, Franklin Roosevelt, in 1932.  However, the Democrats retained their conservative, southern wing until civil rights legislation led white voters to increasingly back Republicans after 1968.
The parallels are not exact this time round. But it is possible to imagine that a convention fight might deny Mr Trump the Republican nomination in favour of a more acceptable candidate to the party's mainstream (John Kasich, or even, Mr Romney himself), That might prompt the disgruntled billionaire to set up his own third party run, in all probability handing the election to the Democrats.