WASHINGTON — Pete Buttigieg said Monday that he raised $24.8 million during the second fundraising quarter, a massive sum that cements him as a top White House contender despite entering the Democratic presidential primary as a little-known Indiana mayor.

The impressive haul tops the $18 million raised last quarter by Bernie Sanders, who led the Democratic field in fundraising during that period. It will help Buttigieg transition from a scrappy startup operation to a more formidable campaign for the 2020 Democratic nomination and give him staying power to weather the summer months, when fundraising typically dries up.

Meanwhile, many of his better-known rivals have struggled to raise money and could face more challenging circumstances.

“He did exactly what you are supposed to do,” said Rufus Gifford, who was finance director of President Barack Obama’s 2012 campaign. “They are seizing on opportunities, they are building infrastructure, hiring staff and working their butts off.”