LONDON – The fraternal fight to lead Britain’s Labor Party culminated Saturday with Ed Miliband, a 40-year-old pro-union darling, narrowly upsetting his older brother to become the new face of the British opposition.

Labor has limped along since May, when it suffered its worst political defeat in decades, forcing its former chief, Gordon Brown, to surrender the keys to No. 10 Downing St. to the conservative David Cameron. Since then, Labor has struggled to find a fresh voice and message as it navigates its role in the opposition for the first time in 13 years.

That voice will now come from Miliband, who topped his better known and more accomplished brother, David, by a margin of 50.6 percent to 49.4 percent in the party’s complex vote tally Saturday.

The younger Ed leap-frogged David, taking what had seemed his preordained place at the top of British politics. Whereas David Miliband — the front-runner Hillary Rodham Clinton once described as “vital” and “attractive” — held the senior job of foreign minister in Brown’s cabinet, Ed Miliband had a comparatively junior role of climate-change chief.

But as his campaign gained strength, he showed himself to be a politician more emotive and more everyman than his overtly ambitious older brother.

Now, it will be the younger Miliband charged with leading, and rebuilding, the Labor Party.

Miliband, who some fear may tilt the party back to the left and away from the “new Labor” centrist line put forward by Tony Blair in the 1990s, came out on top by successfully tapping a segment of the party that had been long ignored: unions.

Labor’s convoluted balloting process grants unions, along with elected Labor politicians and other key constituents, a significant say in who wins the party’s leadership. The younger Miliband — who appealed to the unions more than his generally pro-business brother and three other challengers — secured their overwhelming support. After results were announced, and following the congratulatory embrace with his brother, Miliband vowed to rebuild Labor in a fresh mold.

“I do believe this country is too unequal,” he said. He later added, “We must have a society that upholds and protects things beyond the bottom line.”

He will now go against Cameron in the pits of the British Parliament, where a tradition of verbal jousting will shape Miliband’s image with the public.

He has been more skeptical about the massive budget cuts proposed by the new ruling coalition of Conservatives and Liberal Democrats than his brother, and many say he may more forcefully oppose them now. He has also called for an increase in the minimum wage, higher taxes on the rich and other policies aimed at addressing the growing gap between rich and poor.

But his election might complicate attempts to rebuild the Labor vote, which crumbled under Brown. Opinion polls showed that David Miliband, a photogenic 45-year-old, was better poised for the task, being more popular with the public at large. The elder Miliband was also seen as an heir to Blair, with whom he helped craft Labor’s shift from the left to the middle of British politics.