He is staggering towards the finish line to claim the Republican presidential nomination after a bruising primary battle.

Former Massachusetts Governor Mitt Romney has failed to win a ringing endorsement from his own party, which has almost embraced him as the inevitable nominee. After last week's primaries in three states gave him more than half of the 1144 delegates who will crown the nominee at the party convention in August, Romney's position looks unassailable.

Yet, his main conservative opponent, former Pennsylvania Senator Rick Santorum, has refused to stand down. Despite polls showing his lead is dwindling over Romney in Pennsylvania, which holds a primary on April 25, Santorum is vowing to "charge out of the locker room". On Friday, he was given a pair of boxing gloves by a state legislator.

But increasingly, the party elders are saying "game over", as former President George W. Bush's campaign strategist, Karl Rove, pointed out in the Wall Street Journal.


They realise that the longer the fratricidal struggle between the two party wings goes on, the more President Barack Obama stands to benefit. The media have already moved on: they are now predicting a Romney victory in the primaries, and their focus has now shifted to who will be his running mate.

Ohio and Florida are the likely battlegrounds in November, and some speculate that a vice-presidential candidate should hail from one of those states. But Senator Marco Rubio of Florida has ruled himself out, saying categorically: "I'm not going to be the vice-president." From Ohio, the media have latched on to Senator Rob Portman, described as not an "exciting" candidate but one who could "fix Washington", by journalist John Heilemann in an interview on MSNBC.

He is co-author of Game Change, the best-selling account of the Obama election victory in 2008. The Atlantic's Major Garrett stuck his neck out on Saturday and predicted that Portman would be on the ticket. Unlike Sarah Palin, the Republican vice-presidential candidate in 2008, "the files are ready, and, by Washington standards, spotless", he wrote.

Another alternative would be for Romney to pick a representative of the conservative party base. Step forward Paul Ryan, the 42-year-old chairman of the House Budget Committee who campaigned with Romney in his home state of Wisconsin, which was won handily by the multi-millionaire last Wednesday. Aides spoke of "chemistry" between the two. Romney says it is too early for him to pick a running mate but Obama fuelled the Romney-Ryan ticket speculation by coupling them together in an attack on the House Republican budget.

Romney knows he has his own negatives. He has been accused of flip-flopping on such issues as abortion. Polls show that because of his recent sharp turn to the right in hopes of attracting the party's conservative wing, Romney has lost support among women voters and Latinos. And his Mormon faith remains a stumbling block for the party's Christian evangelicals, who consider it a cult. A Gallup poll says 18 per cent of Republicans say they would not vote for a Mormon for President.