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WARREN, Michigan — Sen. Bernie Sanders, while acknowledging that some Americans "don't like" the health care law and continuing to call for universal coverage, pledged before a crowd of thousands to fight Republican efforts to repeal the Affordable Care Act.

"There are differences of opinion about the Affordable Care Act — some like it, some don't like it. But very few Americans believe that we should repeal the ACA without a replacement program to make it better," Sanders, I-Vermont, said at a rally Sunday, one of several dozen gatherings across the country aimed at ginning up public support for the health care law.

"Know we are saying to our Republican colleagues: We will not allow you to throw up to 30 million Americans off of health insurance," he said.

Sanders appeared alongside Senate Minority Leader Chuck Schumer, D-New York, both of Michigan's Democratic senators and a handful of the state's House members, as well as a number of progressive activists. According to Sanders, Democrats organized 70 rallies Sunday from Maine to California as part of their campaign to defend the law against Republican efforts to dismantle it.

Related: Obamacare: Five Things You May Not Know About the Health Care Law

Thousands gathered in below-freezing weather, some waiting for hours, to see the lineup of Democrats tout the Affordable Care Act. While the event was initially planned to be inside for an audience of 700 to 1,000, it was moved outside to accommodate the bigger crowd.

Getting a bit of déjà vu--here is the line for the @BernieSanders led "Save Health Care" rally in Warren, MI. It's 28 degrees out. pic.twitter.com/RhAAa0A5Lr — Danny Freeman (@DannyEFreeman) January 15, 2017

At times, the atmosphere took on a campaign feel, with attendees showing their displeasure with the incoming Trump administration through colorful signs, including one that read "Donald Trump, America's First Russian President," much to the crowd's glee.

Gary Isham and his daughter Sagen drove an hour to Macomb Community College from Burton, Michigan, to support the law.

"I'm coming out to support my brothers and sisters to make sure we can save heath care for the folks here in Michigan," Gary Isham said.

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When asked how they felt about the state of the health care debate in Washington, D.C., all Sagen Isham could initially muster was, "Scary."

"I have pre-existing conditions. I need the coverage, so I'm here," she added, noting that she had recently signed up to receive coverage from the Affordable Care Act.

"All we can do is hope and pray that [Trump] gets on the same page," her father said.

Related: Six Things You Might Not Know Could Go With Obamacare Repeal

In Buffalo, New York, images showed dozens of protesters gathered outside the local Veterans Affairs medical center, while in San Francisco, a crowd of hundreds assembled at the steps of City Hall, where Rep. Nancy Pelosi, D-California, excoriated Trump and the Republicans and offered the Democratic catchphrase of the moment: "They want to make America sick again," she said, according to NBC Bay Area.

In Aurora, Colorado, more than 100 people arrived at the city's central library expecting a town hall to be hosted by Republican Rep. Mike Coffman. But many of them instead remained outside a community room, unable to speak with him, NBC station KUSA of Denver reported.

Among those seeking Coffman's ear was Berthie Ruoff, a recent widow and cancer survivor who was able to obtain health insurance after her husband's death through the ACA, the station reported.

"I'm trying to get an answer, and I can't even get in,” she told the station, adding: "I am potentially going to lose my health insurance. ... What's going to happen to me?"

Crowd starts singing while trying to get in to see Congressman Mike Coffman in Aurora. He only agreed with people a couple at a time. #9News pic.twitter.com/bqZgj4FF6f — Nelson Garcia (@9NewsEducation) January 14, 2017

While Ruoff and others waited in the library lobby, chanting and singing, Aurora police officers established a perimeter outside with crime scene tape to allow Coffman to leave through a different exit, KUSA reported.

"We were told everyone would get their time," Stephanie Brook Chavez, an Aurora resident, told the station. "I think he couldn't handle it."

Coffmann's spokesman, Ben Stein, said in a statement that Coffman had been scheduled to hold individual constituent meetings and that he met with 70 people, four at a time, for a few minutes a session.

"Unfortunately, we only reserved the room at the Aurora Central Library for 90 minutes, which is usually plenty of time to see everyone," the statement said. "For those who were unable to see the Congressman today we apologize. These constituents are invited to attend upcoming meeting opportunities and we will block more time so that he can hear from more of his constituents."

The House voted Friday along party lines to instruct committees to finish drafting legislation to repeal the ACA by Jan. 27, following a Senate vote in favor of the same measure Thursday. It's the first step in what many experts expect to be a long road to repealing and replacing the law.

Republicans are grappling over whether to repeal and replace the law at the same time — a process that could take months as Republicans work to draft an alternative — or to fulfill their campaign promises by repealing it as soon as possible, even if they don't have a viable replacement plan.

House Speaker Paul Ryan, R-Wisconsin, said Sunday that they're planning to repeal and replace the law on the same day.

Hinting at the battle to come, Sanders said the rally Sunday was "the beginning of the fight, not the end of the fight."

But even as Republicans are struggling to figure out a way forward on health care repeal, Democrats are facing a similarly tricky political landscape in defending the law. Polls largely show Americans split or opposed to the law, even as a wide majority oppose repealing it outright. Democrats, including President Barack Obama, have acknowledged that the law is flawed and must be improved.

Schumer, while targeting Republicans for lacking a replacement plan, said Democrats would be open to fixing parts of the law.

"We're willing to look at making it better, but we sure as hell ain't gonna repeal it — and we're certainly not gonna repeal it when our [Republican] friends there don't even have a plan to replace it with," he said.

And Sanders, who criticized Obamacare during his bid for the Democratic presidential nomination, echoed progressive complaints that the law didn't go far enough in promising a "Medicare-for-all single-payer system" sometime in the future.

"Our job today is to defend the Affordable Care Act — our job tomorrow is to create a Medicare-for-all single-payer system," he said.