Since its passage almost seven years ago, Republicans have campaigned on the promise of repealing the Affordable Care Act.

But now in power in Washington and the Colorado Senate, GOP lawmakers are realizing the challenge of fulfilling that pledge, as Democrats and like-minded activists have begun a counterattack to protect President Barack Obama’s signature law.

The anti-repeal campaign has included several rallies in Colorado, with much of the pressure aimed at U.S. Rep. Mike Coffman, an Aurora Republican who easily won re-election in November but represents a swing district Democrats long have tried to take.

During a celebration Monday for civil rights icon Martin Luther King Jr., Coffman was heckled by protesters who were upset by his vote Friday that laid the groundwork for a repeal of parts of the Affordable Care Act. At one point, a steady refrain of “No repeal” almost drowned out his words.

The chants followed a flare-up during a weekend meeting Coffman took with constituents, a get-together that ended in disappointment for many as Coffman held small conferences rather than a large town hall.

“Activists angry about the impending repeal of Obamacare came with the goal of making a show. That’s their right,” noted a Twitter account run by Coffman’s campaign team. “My resolve isn’t the least bit shaken by these antics — the American people want Obamacare repealed and replaced.”

But liberal Colorado activists have vowed their efforts were only beginning.

Last week, a coalition of health care and labor advocates delivered petitions to U.S. Sen. Cory Gardner’s offices in Denver and Greeley, imploring the Republican to “support health care, not chaos.” On Sunday, U.S. Rep. Diana DeGette spoke at a union hall in Denver alongside state Democratic leaders Lt. Gov. Donna Lynne and House Speaker Crisanta Duran to rally opponents of repeal.

“This is real. This is not manufactured. People are … angry and ready to fight back,” said Ian Silverii, the executive director of ProgressNow Colorado.

The push to dismantle the health care system created under Obama extends to the Colorado General Assembly.

The Republican-led Senate made a repeal of the health insurance exchange one of the first bills of the 2017 session. The measure, Senate Bill 3, phases out the state benefits exchange after a one-year grace period, forcing the 150,000 people who use the state marketplace to buy insurance on the federal exchange.

“I did not agree originally that a state-managed health care system and health insurance system is beneficial to the state or the country,” said Senate Majority Leader Chris Holbert, R-Parker. “I think health care ought to stay a free market solution.”

The bill is unlikely to pass in the Democratic-led House, but critics reacted harshly to the message the bill sent.

“We can’t just take 150,000 people and just chuck them down the drain because somebody wants to dismantle something like that,” said Senate Democratic leader Lucia Guzman of Denver.

Michael Fields, the Colorado director for Americans for Prosperity, a leading critic of the health care law, accused Democrats of misleading people by suggesting a repeal won’t pair with a replacement.

“I think some of it is trying to scare people about something that isn’t a reality,” he said, even as he acknowledged the challenge ahead. “Any big change that anybody is going to do is going to have some kind of backlash.”

How much political pain Republicans will feel — if any — is an open question, though strategists in both parties said the fight has the potential to reshape the landscape.

“I think you’re going to see a significant backlash all across the state,” said Craig Hughes, a top Democratic strategist in Colorado whose firm is organizing opposition to the repeal effort. “I think it’s one thing to say you want to repeal the ACA, but if there’s no guarantee of coverage to millions, that’s a different ballgame.”

David Flaherty, a Republican pollster in Colorado, said it’s possible the politics will boomerang against his party.

“They are really playing with fire,” he said. “I think the Democrats will probably score some points. It’s not easy to replace this thing.”

Republicans, however, are mounting their own defenses. Last week, the right-leaning American Action Network bought two weeks of commercials to tout efforts by Republicans to undo the health care law.

The six-figure buy in the Denver media market is tailored to help Coffman. Ruth Guerra, a spokeswoman for the group, said in a statement that “Americans deserve to know that Speaker (Paul) Ryan and House Republicans are offering a better way forward with a plan to replace Obamacare.”

Throughout the 2016 campaign, Donald Trump and other Republicans ran on the promise that a repeal — and a replacement — of the Affordable Care Act would be a top priority.

Part of that pledge started to take shape last week when congressional Republicans moved a measure through the House and Senate that takes aim at sections of the law that deal with the budget — but not the whole thing.

Republicans don’t have a big enough majority in the Senate to pass a new health care law without Democratic support — a difficult prospect that is complicated by a lack of agreement among Republicans on how they should proceed next.

Trump said he plans to force drug companies to negotiate with the government on prices.

“We’re going to have insurance for everybody,” he told The Washington Post. “There was a philosophy in some circles that if you can’t pay for it, you don’t get it. That’s not going to happen with us.”

While it’s expected that any change will come with a transition phase for consumers to find new insurance, a repeal still puts Republicans on the clock to find a solution — a window that gives Democrats time to turn up the heat.

In Colorado, for example, an estimated 407,000 residents have gained health insurance through the law’s expansion of Medicaid: one reason the number of Coloradans without coverage has dropped by more than half, from 15.8 percent in 2011 to 6.7 percent in 2015.

Colorado hospitals, too, have seen a sharp drop in the cost of so-called “uncompensated care” for the uninsured, from $2.3 billion in 2009 to $1.1 billion in 2015.

U.S. Rep. Ed Perlmutter, D-Arvada, described as worrisome early efforts to undo the law.

“Unfortunately, this is the first step in the Republicans’ plan to repeal the Affordable Care Act with no plan to replace it,” he said in a statement. “Coloradans and Americans deserve better.”

Asked about GOP replacement plans, U.S. Rep. Ken Buck, R-Windsor, talked about reducing regulations on insurance companies to pass along savings to consumers.

But Buck added that it is imperative for Republicans to hear from constituents.

“I don’t pretend to have the best answer,” Buck said. “I want to listen.”

Staff writers John Aguilar and Brian Eason contributed to this report.