When left to voters, raising the minimum wage has had astounding success.

Over the past 20 years, every statewide minimum wage ballot measure but two has passed. In 2014 alone, four states — Alaska, Arkansas, Nebraska and South Dakota — approved a minimum wage increase by ballot measure. And that makes sense: When given a choice, voters will choose the one they think gives them and their neighbors more money.

But these ballot measures could be victims of their own success. Now even the threat of a credible one is enough to spur Republican-led state legislatures to take matters into their own hands to stop it — and any significant wage increase that would come along with it.

The progressive Ballot Initiative Strategy Center is tracking five ballot measures in four states and the District of Columbia that could be thwarted by opponents. This year, state legislatures in Alabama and Idaho passed laws preventing municipalities from raising their own minimum wages. Other states, like Michigan, voted in 2014 to raise the minimum wage, but for far less than what advocates were proposing in a ballot measure. In states where a ballot measure for a wage in the $12 range appears to be a done deal, as  in Maine, opponents are trying to get a competing, lower proposal on the ballot for November.

The latest blow to minimum wage increase advocates comes in Arizona. On Friday, the Republican-controlled state Senate approved a potential ballot measure that would raise the minimum wage to $9.50 by 2020 and prevent cities and towns from raising it more than that in the meantime. It still needs to pass the full state House.

Advocates say the legislation is a direct response to a statewide attempted ballot initiative to raise it by $12 and a local Flagstaff initiative to raise the city's minimum wage to $15 an hour. By moving to increase the wage more modestly and putting a cap on additional increases, opponents of the higher minimum wages can effectively game the system — saying they pushed for an increase while also preventing bigger increases.

Minimum wage fights are as old as labor laws themselves. But these new battle lines are especially frustrating to minimum wage advocates, who in the past five years have started to rely mostly on ballot initiatives to raise wages.

They're certainly not getting much help from federal and state legislatures. Congress has raised the minimum wage three times in 30 years. The last time was in 2007 when Democrats — who were in charge at the time — set in motion a $7.25 minimum by 2009, a number critics note barely keeps up with inflation, if at all.

Minimum wage advocates don't have many friends at the state level, either. Republicans control 69 out of 99 state legislative chambers and are generally not supportive of minimum wage increases.

Not that there hasn't been any legislation on the minimum wage. In 2014, 10 states and D.C. voted to raise their minimum wages. But like what happened in Michigan, many of those states raised it a fraction of what advocates had pushed for.

There is some good news in this story for minimum wage advocates, though. And it's that the mere threat of ballot initiatives can be leverage to get what they want, too.

In Oregon and California, for example, there were rumors last winter of impending ballot initiatives for $15. Instead, Democratic lawmakers acted on their own. Oregon Gov. Kate Brown (D) signed a $14.75 minimum wage into law in March. California Gov. Jerry Brown (D) signed a $15 minimum wage into law in April. And most recently, New York Gov. Anthony Cuomo (D) signed a $15 minimum wage into law, notably with Hillary Clinton by his side.

When voters are about to increase the minimum wage themselves, it seems, Democratic lawmakers would rather just do it themselves and take the credit.

Democratic presidential candidate Hillary Clinton attended a rally with New York Gov. Andrew Cuomo and celebrated the raising of the state's minimum wage to $15 (Reuters)

Mind you, Clinton doesn't endorse a $15 federal minimum wage — something her primary opponent, Sen. Bernie Sanders, has knocked her for — but she says she's fine with it at the state level.

And ballot initiative battles aside, the fact that the likely Democratic presidential nominee is okay with a minimum wage that's more than twice the federal level shows how far minimum wage advocates have come in just a few years.

In 2012, fast food workers in New York City went on strike for a $15 minimum wage. In 2013, President Obama proposed a $9 federal one in his State of the Union (he now wants to see a $10.10 minimum). In 2014, Seattle became the first municipality to approve a $15 minimum wage. Now, two states — California and New York — have matched that, and 29 states and D.C. have a minimum wage higher than the federal level, according to the National Conference of State Legislatures.

But for all their success at changing the political conversation around minimum wage, ballot measures remain minimum wage advocates' go-to method for actually changing minimum wage laws. And this year especially, minimum wage opponents seem to have found a way to block that, too.