The influence that the minimum wage debate had on the Democratic victories eight years ago was overshadowed by other factors. Only about one-third of Americans approved of how then- President George W. Bush was handling his job, including an unpopular war in Iraq and fresh memories of the botched response to Hurricane Katrina along the Gulf Coast.

Missouri, Montana

Republicans might have overcome those hurdles in Missouri and Montana had it not been for the minimum wage issues there. Those victories delivered the party a two-seat Senate majority.

In all, Democrats picked up U.S. House or Senate seats in five of six states where voters approved raising the wage floor in 2006. The measures passed with the highest margins in the two states that mark the start and finish of the Missouri River.

"One of the reasons it was so powerful was that my opponent had voted against it," McCaskill said last week in an interview at the Capitol. "That's a hard one to explain to people — why you would have a principled opposition to any kind of increase in the minimum wage."

The Missouri ballot question was created largely to drive turnout for McCaskill's campaign, said Jim Kottmeyer, a former executive director of the Missouri Democratic Party who led the state's minimum wage campaign in 2006. Internal research showed voters initially were more interested in the wage issue than the U.S. Senate race, Kottmeyer said in a telephone interview.

While business groups, such as the National Retail Federation, lobby against the federal increase this year, there was no organized opposition to the Missouri ballot question. That let the Missouri group spend almost all of its money on turnout operations to help McCaskill, Kottmeyer said.

McCaskill victory

The measure won 76 percent of the vote, while McCaskill edged Jim Talent by 48,300 votes, or 2.3 percentage points.

In Montana's 2006 Senate race, Democrat Jon Tester beat Republican incumbent Conrad Burns by 3,600 votes, or less than 1 percentage point, while 73 percent of voters agreed to increase the minimum wage.

Tester supported the increase. Burns didn't.

Both Burns and Talent had voted against a minimum wage measure in the U.S. Senate just four months before the election.

Doug Mitchell, who organized the Montana ballot question, said the issue was designed without the Senate race in mind, and little was spent on the effort. Still, the issue came up in the candidates' debates.

'So close'

"I don't think it ever hurts to be on the side of something that has 70 percent popularity, particularly among independents and particularly in a race that was so close," Mitchell said in an interview.

One of the first votes Tester and McCaskill took as U.S. senators was for a 41 percent minimum wage increase, to $7.25 an hour. Democrats are proposing this year to lift it again to $10.10, for a 39 percent jump.

Senate Majority Leader Harry Reid postponed a vote on the issue earlier this month to give labor unions more time to organize support, said a Senate Democratic leadership aide who requested anonymity to discuss strategy.

Walden, chairman of the Republican campaign committee, said he changed his position on the increase this year because "the economy was better in '07."

A report last month from the nonpartisan Congressional Budget Office said raising the minimum wage to $10.10 in three steps would reduce employment nationally by about 500,000 workers, or about 0.3 percent. At the same time, the report said the increase would lift 900,000 people out of poverty and add $31 billion in earnings for low-wage Americans.

'Best left'

"This minimum wage setting is best left to the states, for the most part," Walden of Oregon said in an interview. "The economy in New York City is different from the economy in rural Oregon, and that's true all over the country."

Representative Tom Cole of Oklahoma, who oversaw Republican House elections in 2008, said state actions to raise the minimum wage have taken pressure off lawmakers.

"At this point, it doesn't seem to be moving many people," he said of his fellow Republicans. "Politically, they don't feel threatened by it."

House Speaker John Boehner, an Ohio Republican, opposes moving a federal wage increase measure through the chamber, saying last week that the plan "really makes no sense at all."

This year, 21 states and the District of Columbia have set the U.S. minimum wage above $7.25 an hour. Before the last increase, 17 states and D.C. exceeded the U.S. wage floor, Labor Department data show.

Poll support

Multiple polls show that about three-fourths of Americans back minimum wage increases, including one taken in January by McLaughlin & Associates, House Majority Leader Eric Cantor's pollster.

The poll showed that only about one-third of respondents said raising the minimum wage would help their finances. About 60 percent said they'd rather leave the issue to the states.

Democrats are using the minimum wage issue to distract from Obama's poor approval ratings and the troubled rollout of his health-care law, said Senator Jerry Moran, chairman of the National Republican Senatorial Committee.

Americans want to see "Congress working together to fix the problems created by Obamacare," said Moran, who voted for the 2007 increase. "That is of greater consequence than anything else we deal with."

A minimum wage increase is set for a vote in Alaska's Aug. 19 primary and the Nov. 4 election in South Dakota. Similar ballot initiatives have been proposed in Arkansas, California, Idaho, Massachusetts and Missouri, according to the Ballot Initiative Strategy Center.

Pryor's opposition

In Arkansas, Democratic Senator Mark Pryor has opposed his party's plan for a $10.10 hourly wage floor, while backing the state proposal, which would lift the wage to $8.50. His Republican opponent, Representative Tom Cotton, opposes the federal plan and hasn't taken a position on the state one.

Democrats, meanwhile, are attacking Republican Senate candidates in Alaska, Kentucky and Iowa who oppose the latest wage increase as heartless toward single mothers, indifferent to the working poor and anti-middle class.

"The vast majority of the American people support an increase in the minimum wage," Senator Michael Bennet, a Colorado Democrat in charge of his party's races this year, said in a brief interview. Opponents will "be subject to the observation that their priorities are somehow decoupled from the priorities of the American people."

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