Sen. Tom HarkinThomas (Tom) Richard HarkinDem Senator open to bid from the left in 2020 Senate GOP rejects Trump’s call to go big on gun legislation Trump should require federal contractors to follow the law MORE (D-Iowa) said Tuesday that he was “mystified” that Republicans are opposing his bill to raise the federal minimum wage.

Harkin is the leader author of the Fair Minimum Wage Act, S. 2223, which would raise the minimum wage from $7.25 to $10.10 an hour over two years. The bill also increases over six years the tipped minimum wage from $2.13 an hour to 70 percent of the minimum wage. Further wage increases would be indexed to inflation.

The Senate will take the first procedural vote on whether to advance the debate of the bill on Wednesday. Harkin said he hoped at least five Republicans would join Democrats to overcome the first 60-vote threshold hurdle.

“I am mystified by how vehemently my Republican colleague oppose this modest increase — I just don’t understand it,” Harkin said on the Senate floor. “I hope my colleague will do the right thing. Vote for cloture to allow us to get onto the bill.”

Republicans have argued that Harkin’s bill would harm job growth, citing a Congressional Budget Office (CBO) report that says the bill would cost the economy nearly 1 million jobs.

Sen. Susan Collins (R-Maine) has proposed an alternative that would increase the minimum wage to $9 an hour, greatly reducing the negative job effect. But Democrats insist the wage should be $10.10 an hour because that would lift full-time workers out of poverty.

“It’s going to give an economic boost, increase the [Gross Domestic Product] of our country, but basically it’s really about economic fairness and what kind of society we want to be,” Harkin said. “It’s about the core American value that no one who works full-time all year long should live below poverty. … That’s just not fair.”

Harkin said Republican should support his bill because once nearly 2 million full-time workers are lifted out of poverty it would dramatically reduce government spending on safety net programs such as food stamps. Harkin said in the first year alone, his bill would reduce federal spending by $4.6 billion.

Democratic Sens. Tom Udall (N.M.), Jeanne Shaheen (N.H.), Barbara Boxer (Calif.), Tammy Baldwin (Wis.), Amy Klobuchar (Minn.) and Patty MurrayPatricia (Patty) Lynn MurrayIBM-led coalition pushes senators for action on better tech skills training Members of Congress demand new federal gender pay audit Overnight Health Care — Sponsored by PCMA — Health chief grilled on Trump drug pricing plan, ObamaCare case MORE (Wash.) joined Harkin on the Senate floor Tuesday in calling for passage of his bill because it would result in a raise for 28 million workers.

Democrats have made economic equality a central issue to their midterm election agenda as well women's issues.

"This idea could change the lives of millions of Americans if Congress simply acted to increase the minimum wage," Murray said. "One-in-four women are making the minimum wage."

Even if the Senate advances a bill to raise the minimum wage, House Republicans have said that’s not the solution. Instead Republicans are pushing reforms to job training programs, such as the SKILLS Act, and measures to create new jobs such as approving the construction of the Keystone XL pipeline or repealing ObamaCare.