(CNN) As the July 4th congressional recess winds down, activists from a coalition of progressive and leftist political groups are ramping up protests against the Senate health care bill.

The controversial plan, universally opposed by Democrats and now struggling to secure support from within the GOP, would effectively repeal Obamacare and dramatically restructure federal funding for Medicaid. Ten Senate Republicans, some worried about blowback from potential cuts to popular programs and others arguing the bill doesn't go far enough, have said they will not back the legislation in its current form.

Despite growing signs of pessimism on the right, activists spent the extended holiday weekend urging the grassroots to stay engaged, concerned that any lull would provide an opening for Republican leadership to claw back crucial votes. House Speaker Paul Ryan pronounced his chamber's version of the bill dead in late March, setting off premature celebrations in Democratic circles, only to revive and pass it weeks later.

On Thursday, protesters plan to stage a round of coordinated sit-ins at the offices of Republican senators in 21 states. Our Revolution, the political organization spun out of Sen. Bernie Sanders' 2016 Democratic primary campaign, has been promoting the protests in emails to supporters and on its website, where it provides a list of times and locations . Democracy Spring, CREDO, #AllofUs, Democratic Socialists of America, Progressive Democrats of America, and the Working Families Party are among the other groups helping organize the demonstrations.

"We do not see this as political theater," Our Revolution's new president, former Ohio state Sen. Nina Turner, said in an email late Wednesday, "we will continue to sit, stand and fight on until adequate healthcare is available to all."

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