WASHINGTON -- Workers toiling in low-wage jobs marked a dispiriting anniversary on Wednesday: It's now been four years since the last time the federal minimum wage was raised.

The minimum wage is currently $7.25 per hour , and it prevails in the 30 states that don't already mandate a higher one. The last time it saw a boost was on July 24, 2009, when it was raised from $6.55. That 70-cent raise marked the last in a series of increases signed into law by President George W. Bush.

Since then, the legal wage floor has held steady, even as the cost of living has continued to rise. A full-time worker earning the minimum wage now pulls in a salary of about $15,000 per year, far below a living wage in most parts of the country.

President Barack Obama and congressional Democrats have proposed raising the minimum wage and pegging it to inflation, though House Republicans are unlikely to go along with such a hike any time soon. Bills to raise the minimum wage, introduced in the House and Senate earlier this year, have not been voted on by their respective committees. Republican leaders, including House Speaker John Boehner (R-Ohio), have deemed a minimum-wage raise a job killer. (House Republicans have already voted down a version of the minimum wage raise that was inserted as an amendment to a workforce training bill.)

Labor groups supporting a minimum wage boost planned demonstrations in an estimated 30 cities across the country on Wednesday, calling the four-year lapse reason alone for a raise. According to organizers, the protests would target low-wage employers who've seen employee strikes in recent months, including Walmart and McDonald's . A group of companies that back the Democratic proposal, called Business for a Fair Minimum Wage, launched a petition Wednesday for businesses to pledge their support for a raise .

Over the decades, the minimum wage has gone for spells longer than four years before seeing a raise, according to the Labor Department . The wage floor remained stagnant for a full 10 years leading up to the first increase late in Bush's tenure, in 2007. And during the early 1990s, the minimum wage stood at $4.25 for more than five years before being raised under President Bill Clinton.

But if Congress doesn't send Obama legislation to sign by the end of his second term, he will be the first president since Ronald Reagan who didn't raise the minimum wage at all.

Back on the campaign trail in 2008, Obama stumped for hiking the minimum wage to $9.50 by 2011 and pegging it to inflation in perpetuity. That raise never came, however, and the president proposed a more modest boost to $9.00 during his State of the Union address earlier this year. Sen. Tom Harkin (D-Iowa) and Rep. George Miller (D-Calif.) soon rolled out their own, more ambitious proposal, which by 2015 would raise the minimum wage to $10.10, closer to its historical high in the late 1960s.

The proposals by both the president and congressional Democrats include one pivotal measure -- the tying of the minimum wage to an inflation index . Ten states have already undertaken this change, choosing to automatically reset the minimum wage each year rather than wrangle politically over a new raise every few years. If Congress adopted an inflation index, the purchasing power of the minimum wage wouldn't erode as it has for the past four years.

According to a recent HuffPost/YouGov poll , Americans broadly support the idea of raising the minimum wage and pegging it to inflation, with 62 percent of Americans saying it should be boosted to $9. Obama's proposal was supported by a huge majority of Democrats in the poll (86 percent to 6 percent) and a majority of independents (56 percent to 28 percent). Fifty-two percent of Republicans, however, opposed the proposal, compared to 36 percent who supported it.