"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.

In planning sessions and one-on-one meetings with donors, Mr. Ickes, who is a Priorities USA board member, and other Clinton supporters are discussing how to raise as much as $300 million for Democratic outside groups. That is almost twice as much as Democratic super PACs and other outside groups spent to help re-elect President Obama in 2012, when conservative super PACs far outspent liberal ones.

This ambitious goal will require the emergence of a new class of at least 20 Democratic donors who can give $5 million or even $10 million each. Mr. Ickes said recruiting them would not be easy.

“Our side isn’t used to being asked for that kind of money,” Mr. Ickes said. “If you asked them to put up $100 million for a hospital wing, they’d be the first in line.”

The hurdles begin with the candidate. While Mrs. Clinton has committed to meeting personally with potential super PAC donors, people close to her say she has not yet grappled with the kind of big-donor courting that has framed the early months of the Republican race."