WASHINGTON - Democrats and Republicans agree on one thing about the hotly contested Affordable Care Act: When it comes to voter intensity, the GOP holds a clear upper hand.

But three major liberal groups hope to change that in coming months, with plans to spend tens of millions of dollars persuading residents in a dozen key states to vote for Democrats based on the issue. Whether they succeed could help determine not just control of the Senate but the fate of key governor's races and the law's viability.

The Service Employees International Union, Planned Parenthood Action Fund, and MoveOn.org have each launched campaigns in recent weeks aimed at mobilizing support for both the law itself and those officials who back it. By focusing on more popular parts of the law - including Medicaid expansion, free birth-control coverage, and a ban on denying coverage for preexisting conditions - the groups hope to coax individuals who often skip voting in midterm elections to make it to the polls.

The advocacy groups say they are still honing specific plans but the pieces are already in place. Tens of thousands of volunteers and campaign workers are already engaged, having spent the last few months dedicated to signing people up for insurance plans under state and federal health insurance exchanges.

The organizations now plan to apply that same machinery to the midterms, with a particular focus in states that did not expand Medicaid but have competitive gubernatorial or Senate races, such as Louisiana and Florida. They are shifting over thousands of workers and volunteers who had been focused on enrollment to become steeped in Medicaid expansion. And they plan to return to the people they enrolled in coverage and ask them now to vote.

The groups say they are devoting significant resources to the efforts. Planned Parenthood Action Fund will spend between $12 million and $15 million, according to its president, Cecile Richards. MoveOn's PAC and other arms "will spend millions" on issue advocacy and direct support of candidates.

SEIU Healthcare executive vice president Kirk Adams, whose division has more than 1.2 million members, said the Medicaid expansion issue resonates with many voters who often sit out non-presidential election years.