Democratic presidential candidate Hillary Rodham Clinton speaks at Texas Southern University in Houston on June 4. (Donna Carson/Reuters)

An unexpected thing happened over the weekend. Activists and union members convening in Detroit to rally for a $15 hourly minimum wage got a call from Hillary Rodham Clinton, who addressed the crowd by phone.

The former secretary of state did not endorse a $15 minimum wage. But she suggested her allegiance is with the efforts of the rallying workers. "All of you should not have to march in the streets to get a living wage, but thank you for marching in the streets to get that living wage," she said.

Two of her rivals for the Democratic nomination, Martin O'Malley and Bernie Sanders, have already embraced a $15 minimum wage. And so Clinton's decision to call the fast-food workers shows not only the potential power of organized labor in the 2016 campaign, but the way that the $15 minimum wage may become a kind of litmus test for Democratic candidates.

Asked about Clinton's position, the campaign provided the following statement: "She strongly supports workers in the fast-food industry in cities across the country mobilizing to fight for a living wage. In the coming weeks, she will lay out her specific plans for increasing wages."

The level of the minimum wage could prove to be an important theme in the Democratic primary.

"We need more cities and states to follow the lead of Los Angeles and St. Louis and New York," Clinton told the fast-food workers. The Los Angeles City Council recently voted to raise the minimum wage to $15 an hour. The mayors of New York and St. Louis have proposed eventually raising the minimum wage to $15, as well. Other successes for the movement include San Francisco and Seattle.

[What will Los Angeles's $15 minimum wage actually mean for workers?]

Workers at the convention were gratified by Clinton's measured support but said they were aware of the political constraints she confronts as a candidate.

"I honestly believe that Hillary Clinton is trying to use her voice to back up her campaign more than she is to back up our campaign," said Ashona Osborne, 23, an Arby's employee from Pittsburgh. "It's great that she's supporting our movement, but we have to keep in mind that she's willing to do whatever she can to get more votes."

The minimum wage is not the only major economic issue on which Clinton has not outlined a clear position. As Dan Balz reported for The Washington Post over the weekend, Clinton has said little about banking regulation and free trade.

Labor activists oppose the trade deal the Obama administration is negotiating in the Pacific, which Clinton supervised and advocated for as secretary of state. Recently, though, she's stuck to generalities about fairness, workers' rights and economic growth.

Sanders said in a statement that he thought the federal minimum wage of $7.25 an hour was "a starvation wage." He's argued it should eventually be raised to $15 an hour. "That is what I believe every candidate for president should say," he said.

"We don't know where Secretary Clinton stands on the minimum wage," said Haley Morris, a spokeswoman for O'Malley. She noted that O'Malley signed a bill raising Maryland's minimum wage to $10.10 an hour by 2018 while he was in office.

[Here's a simple guide to the real differences between the Democratic candidates for president]

There are reasons that Clinton might hesitate to support a minimum wage of $15 an hour. Most economic research on the minimum wage has focused on the small changes that states and cities have made in the past. No one is sure what will happen if the federal minimum wage more than doubles from its current level. Dylan Matthews has argued in Wonkblog that a $15 minimum wage would be too risky, while Demos's Matt Bruenig gave a rebuttal.

According to some experts, dealing with the issue on a local level might be best anyway, since the cost of living and the average wage vary from city to city.

Clinton is expected to give a series of substantive speeches on policy beginning Saturday in New York.