"Millions of American workers will be watching how each senator votes today. To them, it's a matter of survival," Senate Majority Leader Harry Reid, D-Nev., said before the vote.
He pointedly added, "For Republicans, this vote will demonstrate whether they truly care about our economy."
Republicans, solidly against the Democratic proposal, say it would be too expensive for employers and cost jobs. As ammunition, they cite a February study by the nonpartisan Congressional Budget Office that estimated the increase to $10.10 could eliminate about 500,000 jobs but also envisioned higher income for 16.5 million low-earning people.
"Washington Democrats' true focus these days seems to be making the far left happy, not helping the middle class," said Senate Minority Leader Mitch McConnell, R-Ky.
"This is all about politics," said No. 2 Senate GOP leader John Cornyn of Texas. "This is about trying to make this side of the aisle look bad and hard-hearted."
The vote was 54-42 in favor of allowing debate on the measure to proceed, six votes short of the 60 that Democrats needed to prevail. Sen. Bob Corker, R-Tenn., was the only Republican to cross party lines and vote "yes." Reid switched his vote to "no," which gives him the right to call another vote on the measure. No other Democrats opposed the bill.
Sen. Susan Collins, R-Maine, who has been seeking a deal with other senators on a lower figure than $10.10, said Wednesday that she will continue that effort. Maine Sen. Angus King, an independent who usually sides with Democrats, said he too favors finding middle ground.
But Democratic leaders have shown no inclination to do that a view shared by unions that favor an increase and business groups that oppose one.
"We're not going to compromise on $10.10," Reid told reporters after the vote.
In a clear sign of the political value Democrats believe the issue has, Democrats said they intend to force another vote on the increase closer to this year's elections.
The White House issued a statement urging the bill's passage and saying the administration wants legislation "to build real, lasting economic security for the middle class and create more opportunities for every hardworking American to get ahead."
Supporters note that the minimum wage's buying power has fallen. It reached its peak value in 1968, when it was $1.60 hourly but was worth $10.86 in today's dollars.
The legislation is opposed by business groups including the National Council of Chain Restaurants and the International Franchise Association. The National Restaurant Association has hundreds of members at the Capitol this week lobbying lawmakers on several issues, including opposition to a higher minimum wage.
Also opposed were conservative organizations including Heritage Action and Americans for Prosperity, which is backed by Charles and David Koch. The billionaire brothers are spending millions this year to unseat congressional Democrats, and Senate Majority Leader Harry Reid, D-Nev., and his allies are casting them as unfettered villains.
Other Democratic bills that have splattered against GOP roadblocks this year would restore expired benefits for the long-term unemployed and pressure employers to pay men and women equally. Democrats plan future votes on bills easing the costs of college and child care.
Opposition from Republicans running the House makes it unlikely that chamber would debate minimum wage legislation this year.
According to the federal Bureau of Labor Statistics, about two-thirds of the 3.3 million people who earned $7.25 an hour or less last year worked in service jobs, mostly food preparation and serving.
More than 6 in 10 of those making $7.25 or under were women, and about half were under age 25. Democrats hope their support for a minimum wage boost will draw voters from both groups who usually lean Democratic to the polls in November, when Senate control will be at stake. The GOP's hold on the House is not in doubt.
Harkin's bill would also gradually increase the minimum wage for tipped workers like waiters to 70 percent of the minimum for most other workers. It is currently $2.13 hourly, which can be paid as long as their hourly earnings with tips total at least $7.25.
The minimum wage was first enacted in 1938 and set at 25 cents.
Congress has passed nine laws slowly increasing it, including one each decade since the 1980s. The minimum has been $7.25 since 2009.
Associated Press writer Matt Daly contributed to this story.