WASHINGTON — Over the last few months, Harold M. Ickes, a longtime ally of Hillary Rodham Clinton, has helped organize private meetings around the country with union leaders, Clinton backers and Democratic strategists. The pressing topic: Who will step up to be the Democrats’ megadonors in the 2016 presidential race?

Republican contenders have already secured hundreds of millions of dollars in commitments from a stable of billionaires, including a Wall Street hedge fund executive, a Las Vegas casino magnate, a Florida auto dealer, a Wyoming investor and, of course, the Kansas-born billionaires David H. and Charles G. Koch. But none of the biggest Democratic donors from past elections — for example, the Chicago investor Fred Eychaner, the climate-change activist Tom Steyer and the entertainment mogul Jeffrey Katzenberg — have committed to supporting Mrs. Clinton on nearly the same scale.

“No one has stepped forward as the savior,” said Matt Bennett, a longtime Democratic consultant in Washington.

The leading super PAC backing Mrs. Clinton, Priorities USA Action, has won commitments of only about $15 million so far, Democrats involved with the group’s fund-raising said. And while the absence of a competitive race for the Democratic nomination gives Mrs. Clinton more time to catch up with Republican rivals, her allies are planning to push the party’s wealthiest donors for more money than most of them have ever given.