Republican debate: Marco Rubio reaps benefits of claiming middle ground in GOP presidential race

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Energised by another peppy performance at the fourth Republican debate in Milwaukee, Senator Marco Rubio has brushed off a promised blitz of attack ads against him from backers of Jeb Bush, calling the ploy by his former mentor and friend “unfortunate”.

All the candidates in a still wide-open Republican field left Wisconsin claiming victory over their rivals, perhaps no one more urgently than Mr Bush, the former Governor of Florida, whose unexpectedly limp interventions at the earlier debates had set off a slow leak of air from his campaign.

“I had a good debate because I got to talk about things with a little substance instead of the cute one-liners,” Mr Bush asserted, revealing he had just been endorsed by Senator Bob Dole, the party’s ultimately unsuccessful nominee in 1996.

Steered by moderators from The Wall Street Journal and the Fox Business Channel, the eight contenders who were at the main primetime debate largely eschewed personal attacks, although Donald Trump, still the front-runner in most polls with Dr Ben Carson a close second, was loudly booed for snippily scolding Carly Fiorina, a former Hewlett Packard CEO, for interrupting too much.

But the encounter served to expose the deep divisions in the party both on key areas of policy – notably immigration and foreign affairs – and on the more fundamental question of whether it would be best placed to win back the White House with a candidate promising moderate pragmatism, such as Mr Bush or John Kasich, the Governor of Ohio, or a conservative radical, such as Senator Ted Cruz of Texas.

It may be the special strength of Mr Rubio that he can present himself as representing both camps – balancing a strong anti-abortion stance with more progressive views on immigration.

That he may be emerging as a new favourite for the nomination explains the decision of Right to Rise, the Super-Pac that is supporting Mr Bush, to try to bury him now with what may be as much as $20m’s worth (£13m) of negative advertising. “Ultimately we can’t control other people’s campaigns if they decide that attacking us is the way forward,” Mr Rubio said. “It’s wrong. It’s not going to change my campaign.”

In Milwaukee, Mr Rubio had slick responses to most challenges, even if they sometimes seemed more canned than spontaneous. He repeatedly emphasised his qualification as the only candidate suited for the “21st century”. His best moment was when he stood accused by Senator Rand Paul of Kentucky of wanting to spend too much on defence.

“We can’t even have an economy if we’re not safe,” Mr Rubio said. “There are radical jihadists in the Middle East... a radical Shia cleric in Iran trying to get a nuclear weapon... the Chinese taking over the South China Sea...”

Both Mr Bush and Mr Kasich attempted to burnish their credentials as seasoned and serious leaders with harsh attacks on Mr Trump for promising to deport 11 million people living in the US illegally, branding his plan impractical and contrary to the country’s values.

“That’s the problem with this. The way you win the presidency is to have practical plans,” Mr Bush said.