Advocates for a higher minimum wage are trying to keep the movement's momentum up through a round of state-level ballot initiatives in November proposing a higher rate. They hope that increasing the number of states with high minimums will put pressure on Congress to match it at the federal level.

Voters in Arizona, Colorado and Maine will be asked this fall whether their state's minimum wage should be raised to $12 an hour, while Washington state voters will decide if their rate should climb to $13.50.

Neil Sroka, spokesman for the Howard Dean-founded group Democracy for America, said that the more states raise their rates, the stronger the argument becomes for the federal government to raise the national rate.

"There is no doubt that the ballot initiatives have the effect of keeping the issue on the front burner for Congress. Raising the federal level will ensure that the states are not competing with each other," he said.

The federal minimum wage is $7.25 an hour, but states are free to set their rates above that, and 29 currently do.

The fall's four ballot initiatives continue a newly ramped-up pace for state minimum wage initiates. Five states — Alaska, Arkansas, Illinois, Nebraska and South Dakota — had increases on their ballots in 2014 and New Jersey had one in 2013. Prior to the Garden State vote, the last time any state had a ballot initiative proposing an increase was 2006.

"There's [been] a fair amount of activity on this front compared to previous election cycles," said Jackson Brainerd, policy associate for the nonpartisan National Conference of State Legislatures, which tracks legislation.

If all four initiatives pass, that would put the number of states with minimum wages at or set to go above $10 an hour at 11 counting the District of Columbia.

"It keeps the pressure up and helps to normalize the $15 rate. I think you'll see a lot more pressure at the federal level for an increase," said Michael Saltsman, research director for the conservative Employment Policies Institute.

It already has had an effect, Saltsman notes. As recently as 2014, the Democratic Party was pushing for a federal increase to just $10.20 an hour. Now a $15 rate is part of the party platform.

Activists hit on the state and local-level strategy after it became apparent there was little appetite for an increase in Congress. Their most notable successes came with the California and New York state legislatures, both of which adopted a $15 rate earlier this year.

Fans of a higher minimum wage generally dispute the argument that it has any negative economic effects, pointing to the fact that past predictions of economic harm when rates were raised turned out to be overblown. However, those increases were far more modest than the current $15 rate push.

As some states raise their rates, the others that have kept their rates at the federal level likely will have an easier time of attracting businesses thanks to lower labor costs. Activists are hoping that congressional lawmakers will feel compelled to raise the federal rate in the hopes of keeping the states with high rates from hurting economically.

"There are always going to be a handful of deep, dark red states that will do anything to recruit employers who will pay less than a living wage," Sroka said.

The economic impact of a $15 rate is hard to gauge. There is no precedent for a rate that high, economists note. The states and localities that have adopted it haven't fully implemented it, so there is little data.

A July study by the University of Washington of Seattle's increase to $11 an hour — the rate is set to rise to $15 by 2021 — found that the region's low-income workers were marginally worse off because of it.

"In sum, Seattle's experience shows that the city's low-wage workers did relatively well after the minimum wage increased, but largely because of the strong regional economy. 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 report said.