Sen. Elizabeth Warren (D-Mass.) urged a crowd of progressives to fight for solutions on poverty and student debt, and vowed to reverse the Hobby Lobby Supreme Court decision. She made the comments at the Netroots Nation conference in Detroit. (Netroots Nation)

Sen. Elizabeth Warren (D-Mass.) urged a crowd of progressives to fight for solutions on poverty and student debt, and vowed to reverse the Hobby Lobby Supreme Court decision. She made the comments at the Netroots Nation conference in Detroit. (Netroots Nation)

On the night before her Friday keynote address to a gathering of progressive activists here, Sen. Elizabeth Warren tried to slip into a hotel restaurant for a quiet dinner. But the former law professor has become a liberal superstar, and when a few admirers spotted her walking to the corner of the dining room, they cheered loudly. A moment later, more joined in the applause. Then one urged her, “Run for president!”

The next morning at Netroots Nation, where Warren (D-Mass.) gave a fiery sermon for economic populism — “The game is rigged, and it isn’t right!” — scores of swooning supporters wore faux-straw boater hats with “Warren for President” stickers and chanted, “Run, Liz, run!”

Even as Hillary Rodham Clinton looms as the overwhelming favorite for the 2016 Democratic presidential nomination, the party’s base is stirring for a primary fight. There’s a pining for someone else, and a medley of ambitious Democrats are making moves — many of them previously unreported — to position themselves to perhaps be that someone.

In stark contrast to the overt maneuvering on the Republican side, the 2016 Democratic presidential sweepstakes has been largely frozen in place as Clinton decides whether to run. But with the former secretary of state’s book-tour stumbles exposing a serious vulnerability with grass-roots voters, small cracks are beginning to emerge.

Sen. Amy Klobuchar (Minn.) will test her folksy politics next month in Iowa, home to the first-in-the-nation caucuses. Sen. Kirsten Gillibrand (N.Y.) is coming out this fall with a book, “Off the Sidelines,” that is part political memoir, part modern-feminist playbook and certain to generate presidential buzz. New York Gov. Andrew M. Cuomo also is publishing a memoir this fall with a wink-wink title: “All Things Possible.”

Meanwhile, Maryland Gov. Martin O’Malley seems to respond yes to every party speaking invitation that comes his way and is slated to address Democrats in Nebraska and Mississippi in coming weeks. He also endeared himself to liberals in recent days by breaking with President Obama on how to deal with an influx of unaccompanied minors crossing the U.S. border.

Vice President Biden is making the rounds this summer rallying key Democratic constituencies and recently spoke on a conference call with his former aides — among the hundreds of Biden alumni that date back to his first Senate campaign in 1972. The call was ostensibly just to say hello, but it keeps his political circle engaged.

During a recent vacation in Kiawah Island, S.C., Biden reconnected with old political friends. He played golf with Dick Harpootlian, a former state party chairman, who suggested that Biden is far more “authentic” than Clinton.

“I said, ‘Mr. Vice President, I’ll drive the golf cart,’ ” Harpootlian recalled. “And he said, ‘No, no, no. . . . I’m driving this freaking golf cart. Move over.’ There are some people in this world who like to be driven and some people who like to be in the driver’s seat.”

Itching to build a national network of his own, Missouri Gov. Jay Nixon is heading to Aspen, Colo., next month with O’Malley for a retreat for major party donors. Nixon, who has said he would back Clinton, also recently said that the 2016 field could use a candidate from the heartland who, like himself, gives voice to blue-collar concerns but has red-state appeal.

Sen. Bernard Sanders (I-Vt.) has teased the possibility of a long-shot challenge to Clinton with trips to Iowa and New Hampshire — both early voting states — and plans to return to Iowa for three town hall meetings in September.

One Democrat who knows a thing or two about insurgent campaigns, former senator Gary Hart of Colorado, said he intends to huddle with California Gov. Jerry Brown at their upcoming Yale Law School reunion (Class of 1964) to chat about the possibility of Brown running for the White House.

“Don’t rule out my law school classmate,” said Hart, who ran unsuccessfully for president in 1984 and 1988. “If you pay attention to his career, you see that he does very unexpected things.”

Hart added that Clinton is cautious “politically and personally, and in every way. I think her caution on Iraq cost her the nomination [in 2008]. She’s always trying to find the mythical center on controversial issues — and if you do that, someone else is going to take the bouquet for courage.”

The driving force behind the Democratic maneuvering is a yearning among progressives for a candidate who will champion their economic populist agenda. Anna Galland, executive director of the liberal group MoveOn.org, said income inequality will be the driving issue for the base, just as the Iraq war was in 2008.

“Our members don’t want to see their preferred candidates going to give speeches to big Wall Street banks,” Galland said, a reference to Clinton’s paid speaking gigs, including one next week to a group of financiers in Boston. “They want to see them talking about inequality.”

Although Clinton turned down an invitation to Netroots, she has sought to seize on the issue in other venues. She began talking this spring about “the cancer of inequality” and told television host Charlie Rose on Thursday that if she runs she would offer a detailed agenda “to tackle [economic] growth, which is the handmaiden of inequality.”

Bill and Hillary Clinton are paying close attention to Warren’s rise, said former Pennsylvania governor Ed Rendell, “but they are sagacious enough to understand that Elizabeth Warren couldn’t raise the money” to halt Clinton’s ascent.

Former Vermont governor Howard Dean said he lost his presidential race in 2004 because Democrats “didn’t want to take a chance on the hellfire and brimstone guy.” Dean said he thinks history will repeat itself.

“There will be a primary, and there is always grousing,” said Dean, who insists that he has no intention of running again. “But Hillary, who most Democrats believe has earned it and paid her dues, would have to totally implode in order for a grass-roots candidate to win the nomination.”

Even Clinton’s skeptics acknowledge the difficulty of derailing her juggernaut. If they can’t defeat her, their goal is to shape the debate and pull Clinton to the left on issues such as toughening regulations on Wall Street, expanding Social Security benefits and easing student loan debt.

Warren, with her populist pitch, sharp rhetoric and authentic presence, is the biggest potential threat to Clinton. But although she has insisted that she is not running for president, she is doing some of the things a person running for president does.

Warren published a book this spring, “A Fighting Chance,” and is an in-demand surrogate in the run-up to November’s midterm elections — stumping for Senate and gubernatorial candidates in blue states and red states alike and raising more than $2.6 million for Democratic candidates.

But she is not doing behind-the-scenes spadework expected for a White House run. When she headlined the Minnesota Democratic-Farmer-Labor Party’s Humphrey-Mondale Dinner in March, Warren did not take down names and numbers of the people she met. She traveled with only one aide, hitching a ride from the airport from a local party official, said Corey Day, the party’s executive director.

“There was no advance guy making sure the room was exactly right and her water was cold,” Day said. “You didn’t sense an urgency for her to build a political operation. It was just her and her message, all very low-key.”

By contrast, O’Malley has been getting acquainted with organizers in early-voting states in addition to frequent trips. “He’s all over,” said Raymond Buckley, chairman of the New Hampshire Democratic Party. “He has built up significant goodwill.”

Klobuchar also has kept her calendar full, getting positive reviews for speeches to Democrats in Iowa, Nevada, North Carolina, Pennsylvania and Texas. On Aug. 23, she will return to Iowa to stump for Senate nominee Bruce Braley, aides said.

But Klobuchar has been careful to signal that she wouldn’t run against Clinton, signing up last month to fundraise for Ready for Hillary, the pro-Clinton super PAC.

Hart said it is foolish for Democratic hopefuls to allow Clinton’s indecision to stunt their ambitions. “What are they afraid of?” he asked. “Losing a chance to be in Clinton’s Cabinet? If that’s part of your thinking, you shouldn’t even think about running for president.”

More than anyone else, Warren is speaking directly to the hopes of Democratic activists, who have grown disenchanted with Obama and hope his successor will be a strong progressive change agent.

Here at Netroots, Warren has railed against the influence of banks and corporations, which she said have too many “lobbyists and lawyers, and plenty of friends in Congress.”

“We can whine about it, we can whimper about it, or we can fight back,” Warren said, punching her fist in the air. “I’m fighting back!”

The crowd went wild and screamed for her to run for president. Warren, beaming, tried to hush them so she could carry on with her speech.

Later, when reporters asked about the 2016 buzz, Warren said she is focused on the fall midterms: “We can’t get distracted from that.”

One thing made clear by the scene in Detroit — and others like it recently from West Virginia to Oregon — is that candidate Clinton would be running against Warren in the primaries whether or not the Massachusetts senator enters the race.

“This primary will be about the Wall Street wing versus the Warren wing of the party,” said Charles Chamberlain, executive director of Democracy for America, a liberal group that spun out of Dean’s 2004 campaign. “The question is, will Hillary be with Wall Street like she’s been all along or will she evolve like the party to be with the Warren wing?”

Costa reported from Washington. Alice Crites in Washington contributed to this report.