Barack Obama’s intervention in the event that has overwhelmed coverage of the presidential election in the US, the tragic shooting of an unarmed Florida teenager, brought an angry response from the shooter’s father.

“I never foresaw so much hate coming from the president” and other black leaders, said Robert Zimmerman, whose son George shot and killed Trayvon Martin in late February while apparently on patrol for neighbourhood watch in the city of Sanford.

Mr Obama’s call for an inquiry into the shooting – saying “if I had a son, he would look just like Trayvon” – infuriated his media critics who accused the president and his supporters of playing racial politics with the case.

Top Republicans, by contrast, and the party’s leading candidates for 2012 with the exception of Newt Gingrich, have avoided largely incendiary rhetoric and echoed Mr Obama in querying why the police did not arrest the killer.

The Republican restraint may reflect the evolution of debate over race in a country with a black president and natural caution in commenting on a case in which key facts remain in dispute.

Randall Kennedy, a Harvard law professor who has written extensively on race relations, said the outcry over the teenager’s death was praiseworthy in a society that had habitually “devalued blacks’ lives”.

“Within a couple of weeks, there was a massive uproar over why the police had failed to make an arrest,” he said.

But debate over the case is also taking place against the continuing Republican primary battles, in which the dominance of white voters is as steadfast as it has been in years.

“The contemporary Republican party is built almost exclusively around white voters,” said Ruy Teixeira of the Centre for American Progress.

Based on exit polls, the National Journal calculated that white voters, especially elderly ones, had cast 90 per cent, and often more, of the ballots in the primary contests, a figure unchanged since 2008.

“From one direction, the Republican presidential primaries have witnessed an epic failure by the ­contenders to attract and engage minority voters,” said Ronald Brownstein in a recent column in that magazine.

Mr Obama’s problem is the flipside of this – he has fared relatively poorly among white voters and increasingly relied on minorities, such as black and Hispanic voters, for his support.

Mr Obama’s campaign’s emphasis on the tentative revival of manufacturing in the swing states of Michigan, Ohio and Pennsylvania aims to reverse his slide among the group that likes him least – white working-class men.

Mr Kennedy says he thinks that “Republicans are torn” about whether to try to win over African-Americans, who voted overwhelmingly for the Democrats long before Mr Obama came along.

“Failure presupposes an effort on the part of Republicans to do something about it,” said Mr Kennedy. “Perhaps they don’t want a whole lot of black voters because it would complicate things for them.”

Michael Barone, of the American Enterprise Institute in Washington, said that voting in ­primaries should not be over-analysed, because the turnout “is ginned up by the ­candidates, not by the ­parties”.

“But if whites are on the path to becoming a minority, it could be they will start to behave like a minority” and mainly vote for one party, he said.

However, if there has been a re-racialisation of politics along the lines of blacks and whites, Republicans still hope they can regain ground with Hispanics, the fastest-growing minority in the country.

The Democrats hold a substantial lead in polling of Hispanic voters, largely because of Republican attacks on immigration that swelled with the departure of George W. Bush, who successfully courted ­Latinos.

But the rise of Marco Rubio, the Florida Republican senator whose family hails from Cuba, as a ­potential running-mate for Mitt Romney embodies ­the hopes of many conservatives that the party can start to win Hispanics back.

“Parties adjust in different ways,” said Mr Barone, “and the Republicans are adjusting to the Hispanic vote.”

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