As recently as last year, the Democratic Party's leaders thought the idea of a $15 federal minimum wage was unrealistic and even pushed back against it.

That ended in July when the official party platform, bowing to pressure from organized labor and Sen. Bernie Sanders and his supporters, adopted a call for a $15 federal minimum, more than double the current rate of $7.25 an hour.

With Democratic presidential candidate Hillary Clinton leading in most polls, it is likely the issue could come before Congress as early as next year. Advocates for a $15 federal minimum wage will be holding a major rally in Richmond Saturday, culminating in a march to the Virginia capitol to continue the push.

No one can say with certainly what such a massive increase in the rate would mean for the economy. There is no precedent for an increase that high, liberal and conservative economists say.

"Because there has never been an increase of this magnitude, there is nothing to compare it to. As a nation, we have never contemplated a doubling of the national minimum wage. We have always just nibbled at the edges," said Douglas Holtz-Eakin, who served as Congressional Budget Office director from 2003-05 under President George W. Bush.

The evidence that does exist, though, is not encouraging. When Seattle increased its minimum to $15 in 2014, a boost of nearly $6 an hour, it also commissioned a study of the increase's effect by the University of Washington. A report released in July found that an increase to $11 an hour in April 2015, the first part of the phase-in (the $15 rate will not go into effect until 2017), had not benefited Seattle's low-wage workers.

Instead, it resulted in modest declines to their employment rates and hours worked. While the workers did experience boosts in earnings overall, the report found the increases would have happened without the higher minimum thanks to the region's growing economy.

The higher rate in fact probably reduced the gains that they would have otherwise received. The city's employment rate was 4.3 percent in April 2015 when the $11 rate went into effect. By May of this year, Seattle unemployment had climbed to 4.8 percent.

"Seattle's low-wage workers would have experienced almost equally positive trends if the minimum wage had not increased. Although the minimum wage clearly increased wages for this group, offsetting effects on low-wage worker hours and employment muted the impact on labor earnings," the study found.

Professor Michael Long, one of the study's co-authors, told the Washington Examiner that the increase in the minimum wage appeared to slow down the increase in the wage rate for low-income workers relative to the rest of the state. The new rate made many people too expensive to hire or retain.

"The workers became less valuable to their employers," he said.

The report found that the new minimum wage "slightly reduced the employment rate of low-wage workers by about 1 percentage point" while causing the hours worked by them to lag behind regional trends, by an average of about 19 minutes a week.

Seattle is not perfectly representative of the rest of the nation, Long cautions, so the study should not be seen as a clear example of what would happen to the national economy if a $15 rate were adopted. The main difference is that Seattle has been much more prosperous than the rest of the nation. The city's growth rate was triple the national average from mid-2014 through the end of 2015.

That enabled it to absorb the effects of the wage increase with only modest negative impacts. In effect, Seattle's rising economy and its minimum wage hike canceled out each other — and that was well before it reached its eventual $15 rate. Less prosperous regions would not be so fortunate under a blanket federal increase.

Holtz-Eakin, who is president of the right-leaning American Action Forum think tank, warns that an increase of the magnitude being proposed could have serious consequences for the national economy. Employers would have to make up for the higher labor costs elsewhere. That's likely to mean layoffs, less new hiring, fewer hours, more automation or some combination of all four.

"The thing that is lost in the debate is that there is no new money," Holtz-Eakin said. The Seattle example shows that the people likely to be hurt the most are the ones the increase is meant to help. They're the ones whose jobs would be in the most jeopardy of cuts by employers.

Liberal economists say it is too soon to draw negative conclusions, pointing to the lack of hard data. David Cooper, senior economic policy analyst for the Economic Policy Institute, said the reluctance to speculate on the impact of a $15 federal rate was because "most economists are data junkies [and] we haven't seen a real, laid-out plan. Even the Hillary campaign hasn't put out the actual specifics."

They're not doing much work trying to guess, either. "We really haven't done anything that looks at $15 nationally," Cooper said. The institute has been a cheerleader for the issue, however, producing numerous analyses touting the number of people who would have their wages raised. It even hosted a petition on its website endorsing a $15 statewide rate for California.

The Center on Budget and Policy Priorities, another prominent liberal economic think tank, declined to make any of its experts available for an interview.

Some economists may be afraid to address the issue. writer Dylan Matthews tweeted in April: "One really fascinating phenomenon: left-wing economists saying off the record that $15/hr is super-dangerous, but not saying that publicly."

Some left-of-center economists did make the case against it last year. Brookings Institution economist Harry Holzer wrote an op-ed for Fortune in July 2015 endorsing an increase to $10 an hour but expressing "serious worries" about a $15 rate.

"In cities like Seattle, with a relatively more educated workforce and dynamic labor market, it might be a gamble worth taking. But in other cities, such as L.A. and Washington, D.C., with their large populations of less-educated workers, including unskilled immigrants, such increases are extremely risky," Holzer wrote.

Alan Krueger, former chairman of President Obama's Council of Economic Advisers, made a similar argument in an op-ed for the New York Times in October. "Economics is all about understanding trade-offs and risks. The trade-off is likely to become more severe, and the risk greater, if the minimum wage is set beyond the range studied in past research."

Clinton is ambivalent on the issue. She has advocated an increase to $12 an hour, still a huge hike by historical standards, and has not formally endorsed a $15 national rate, though she has argued that states and cities should be able to adopt it, if they can afford it.

"I support the local efforts that are going on that are making it possible for people working in certain localities to actually earn $15," Clinton said during a New Hampshire campaign stop in July.

As recently as 2014, the Obama administration backed a bill putting the federal rate at $10.25 an hour. And when it increased the minimum wage requirement for employees of federal contractors, it put the rate at $10.10 an hour. Last year it came out in favor of a top rate of $12 an hour, which remains its official position, though Labor Secretary Tom Perez, a close union ally, has used his office to help promote a $15 rate.

Liberal activists promoting the $15 rate rarely engage the question of what such an increase would do for the economy. They point to various studies, though most don't back up their claims. Sanders, for example, has pointed to a January study by the University of Massachusetts at Amherst titled, "A $15 U.S. minimum wage: How the fast-food industry could adjust without shedding jobs."

The study argued the restaurants would benefit from increased sales and less turnover but also that the labor costs likely would require price increases and lower profit margins — a difficult scenario for fast-food joints, many of which operate at the margins.

Activists point out that previous predictions of economic doom when the minimum wage was increased didn't pan out, so the current concerns should be dismissed, too. That ignores that prior increases were generally small. Prior to 2007, the largest year-to-year increase in the federal rate was a 50-cent hike in 1996.

The last increase, which took the rate from $5.85 to $7.25, began in 2007 and was fully phased in by 2009. That was the largest increase in history and it raised the rate by $1.40, a 24 percent increase — a far cry from the more than double advocates are calling for now.

"The business lobby has been reading off the same script since the 1880s when it comes to labor standards. That it is always going to hurt the people it is intended to help. It is always going to drive businesses out of town ... It simply has never happened," said David Rolf, a labor organizer with Service Employees International Union and author of The Fight for Fifteen: The Right Wage for a Working America, during a July forum hosted by the Economic Policy Institute.

"The arguments against higher wages and better labor standards seem to be completely resistant to data in a way that almost no other political or social subject matter might be."