Despite labor unions’ disappointments with other aspects of the Affordable Care Act, over the past year, they have been aggressively advocating  for states to opt into one of the law’s main provisions: the Medicaid expansion. Recent public opinion polling suggests that, in the face of widespread resistance to expansion by Republican governors and legislatures across the country, unions’ and their Democratic allies’ emphasis on this issue may have become a powerful campaign issue in the 2014 elections.

First things first: what is Medicaid, and how does Obamacare expand it? Congress enacted the Medicaid program in 1965 to provide basic “health insurance coverage to people with low incomes,” including low-income “pregnant women and children; certain categories of low-income parents and caretaker relatives; and the elderly or disabled “who qualify for Supplemental Security Income (SSI) benefits based on their low income and resources.” Prior to the Affordable Care Act, states were “not required to participate in the Medicaid program,” but all of them did.

As originally passed, the Affordable Care Act mandated that states expand the program to cover “everyone earning less than 138 percent of the federal poverty line (about $15,000 for an individual or $32,500 for a family of four).” The law included generous terms for states to facilitate this change: the federal government pays the full cost for the Medicaid expansion for three years and 90 percent of the cost after that, by contrast to the 57 percent of the program’s expenses the federal government had typically covered prior to Obamacare. Yet, as a result of the Supreme Court’s decision in National Federation of Independent Business v. Sebelius, “a state can refuse to participate in the expansion without losing all of its Medicaid funds; instead the state will have the option of continu[ing] its current, unexpanded plan as is.”

Consequently, as the Associated Press notes, “[p]artisan politics [have been] coloring governors’ decisions on whether to expand Medicaid in their states.” While every Democratic governor in the country has embraced the expansion, “[a]t least 20 [Republican] governors have declined the offer.” As health care reporter Jonathan Cohn notes, the result of this trend “will almost surely be fewer people in Republican-leaning states getting health insurance.” Unions and their Democratic allies have seized on this issue. For example, the SEIU has engaged in an extensive campaign to pressure Republican Governor of Pennsylvania Tom Corbett to embrace the Medicaid expansion. In Florida, the SEIU and allies including AFSCME have been organizing voters to tell the Legislature that “approving Medicaid expansion will save lives, create jobs and strengthen Florida’s economy.” And in Tennessee, the AFL-CIO Labor Council responded to Republican Governor Bill Haslam’s resistance to expansion in his recent State of the State address by calling on its members and supporters to “elect government officials” in 2014 who support expanding Medicaid for the “hundreds of thousands of Tennesseans who lack quality health care.”

These unions and their allies appear to be on very solid political ground. While public opinion polls on Obamacare generally have been fairly closely divided over the years since its passage, views on the Medicaid expansion are decisively positive, even in Republican-dominated states and among conservatives. Over 60 percent of residents in Alabama, Georgia, Louisiana, Mississippi, and South Carolina – which have resisted Medicaid expansion and are among the most conservative states in the nation – support expanding the program, including nearly half of the self-identified conservatives polled. In Kentucky, where the Democratic governor has expanded Medicaid despite fierce opposition by Kentucky’s two U.S. senators, who are Republicans, “60 percent of self-identified Republicans said they support expansion.”

Many of these states feature major gubernatorial or U.S. Senate campaigns this year, and Medicaid expansion is fast becoming a major issue. For example, Louisiana Senator Mary Landrieu, a Democrat up for reelection in 2014, has “attacked Republicans for refusing to implement the Medicaid expansion.”  In Georgia, gubernatorial candidate Jason Carter, a Democrat, has strongly criticized Governor Nathan Deal, a Republican, for his poor “stewardship” of the state for refusing to expand Medicaid. According to Jennifer Duffy, “who analyzes statewide races for the nonpartisan Cook Political Report . . . Medicaid expansion is just the sort of issue that liberals can use to mobilize” their base in 2014 elections – meaning unions’ and their Democratic allies’ heavy emphasis on this element of Obamacare is likely to continue, and perhaps expand, as campaign season enters full swing.