He has always been the frontrunner but can he convince Americans that he is a credible replacement for Mr Obama?
Plenty of primaries remain to be settled in the US Republican party's presidential nomination race. Among them is the largest prize of all, California on 5 June. Before then, there is a showdown on 24 April in Pennsylvania, home state of and thus a must-win contest for the conservative former senator Rick Santorum. Yet Mr Santorum's failure to win any of the three latest primaries this week, particularly in politically embroiled Wisconsin (where the polarising Republican governor faces a union-driven recall election in June) sends an unmistakable message. That sound you can hear, one commentator said on Wednesday, is the fat lady singing. Unless Mr Santorum can mount an improbable comeback in the next few weeks, the race is over. Barack Obama's Republican challenger in November will be the former Massachusetts governor Mitt Romney.
Mr Romney's win is in many ways no surprise. He has always been the frontrunner. His challengers have come and gone. He is one of the most experienced candidates. He has the immense advantage of a vast personal fortune, accumulated during his years with the private equity company Bain Capital. He is the beneficiary (thanks to the unlimited donations sanctioned by the US supreme court in 2010) of a colossally wealthy funding body – or "Super Pac" – that is wholly dedicated to his candidacy and which has been able to outspend his opponents at every stage in the race. He is also, it is important to note, the Republican candidate who polls best against Mr Obama. Mr Romney should not be underestimated.
In that sense, the US Republicans have again defied the strong impulse to pick the candidate who plays best with the party's militant conservative base. They have gone, instead, for the candidate who seems best placed to appeal beyond the Republican ramparts, to swing voters and independents, just as they did in 2008 by choosing John McCain. It is a choice which proves that pragmatism is not wholly dead within the party. But it is a choice which has been made at a high price. During the primaries, Republican voters have time and again had the opportunity to rally behind Mr Romney and not done so. In state after state, the majority of Republicans have not voted for him. It happened again in Wisconsin and Maryland this week. This is a candidate whom his party manifestly does not love.
Mr Romney has had to fight very hard to get the nomination. Without his money and the ineptitude of his challengers it is questionable whether he would have done so. He is probably the least worst of the candidates the party could have chosen. But he faces an even harder battle now, first in persuading his party to rally behind him, and then in convincing Americans that he is a credible replacement for Mr Obama.