As Democrats in Congress try to bridge differences on healthcare reform legislation, grassroots groups will converge on Washington Thursday to commemorate the 44th anniversary of the passage of Medicare and urge members of Congress to extend its coverage to everyone.

Although most unions are fighting to make sure that the proposed legislation includes a public option, mandates employers to provide or at least contribute to employees’ insurance, and avoids taxing health insurance benefits, unions will be strongly represented in the push for a single-payer (or Medicare-for-all) solution.

560 labor organizations, including 39 state federations and 130 central labor councils, have now endorsed the Medicare-for-all bill introduced by Michigan Democratic Rep. John Conyers. (The bill is formerly called The United States National Health Insurance Act; a parallel Senate bill was introduced by Sen. Bernie Sanders, I-Vt.) While a few unions are pushing for single-payer—most forcefully the California Nurses Association (CNA)—and others have formally endorsed the idea, nearly all are promoting some variant of the Congressional Democratic proposals.

Although systematically excluded from the health reform debate, Medicare-for-all inspires deep passion among supporters as the best policy to guarantee universal healthcare (as even Obama acknowledged in his latest prime-time press conference) and drastically control costs.

Not even most supporters expect there’s much chance single-payer could win in this session, but they want the public and politicians to recognize its merits. So New York Rep. Anthony Weiner plans to introduce an amendment to substitute HR 676 for the House tri-committee bill in the Energy and Commerce Committee, perhaps on Friday. And Sanders has promised to take similar action on the Senate floor.

But the CNA and many labor single-payer advocates are focusing on preserving Ohio Democratic Rep. Dennis Kucinich’s amendment granting states the right to adopt single-payer plans as an alternative to setting up insurance exchanges. Kucinich’s amendment passed the Education and Labor Committee on July 17 by a 25 to 19 vote, with strong Republican support and mainly Democratic opposition.

“If the door [to single-payer] is going to be closed in this Congress,” says CNA national legislative and political director Joe Jurczak, “let’s see if we can pry it open with the state option.”

Canada’s single-payer system emerged from provincial initiatives, and California’s legislature has voted twice to set up state single-payer insurance, only to be vetoed by Gov. Arnold Schwarznegger.

Many labor single-payer advocates doubt that the public option, which is the focus of much union advocacy, will be strong enough to justify support for a bill that they see as deeply flawed. “There’s already a public option that works–called Medicare,” Jurczak says. “Why not extend it?”

“There’s a real possibility we could do more damage” with the Democrats’ “hybrid bill,” argues Kay Tillow, organizer of the All Unions Committee for Single Payer. “At best it’s a plan that doesn’t solve the problem.”

Mark Dudzic, organizer for the Labor Campaign for Single-Payer Healthcare, a coalition formed last winter, said that “the decision still has to be made whether these [Democratic] bills are something we don’t want to stand in the way of or they’re so horrible for working people we have to oppose them.” Dudzic doesn’t want to be accused of blocking reform, but he worries that a bad plan with a weak public option could end up discrediting the public role in healthcare. “It’s a set-up for failure,” he says.

Beyond influencing the debate in Congress, labor single-payer advocates hope to sharpen the discussion on health policy in the labor movement, starting with resolutions for the AFL-CIO convention in September. Then they will persevere, Tillow says, to “build the movement until it’s politically possible” to win their goal.