By Rebecca Shabad, CBS News–

As the new Congress is set to convene Tuesday, Democrats are mounting an aggressive messaging campaign against Republican efforts to repeal Obamacare.

Democrats are warning that repealing President Obama’s signature healthcare law would lead to tens of millions of Americans losing their healthcare coverage and affect Medicare and Medicaid. Since Republicans will have the power to dismantle the major pillars of the law, Democrats are trying to protect it by touting all of its benefits.

“This is not just about the 20 million previously uninsured who now have health security,” House Minority Leader Nancy Pelosi, D-California, said during a call with reporters Monday. “But this is about the 75 percent of American people who get their health benefits through their workplace — not through the ACA.”

Mr. Obama is scheduled to to make a rare visit to Capitol Hill Wednesday morning to huddle privately with House and Senate Democrats to strategize how to preserve Obamacare. Vice President-elect Mike Pence is also heading to Capitol Hill that day to meet with Republicans about their plans.

Watch: Obama meets with Democrats about saving Obamacare

Early this year, Republicans are planning to immediately begin work on repealing Obamacare’s major parts through the budget reconciliation process, which only requires a majority for passage in the Senate and not a supermajority. Last year, the GOP-controlled House and Senate sent a bill that would have repealed the law to President Obama, which he quickly vetoed. That effort could now become reality with Donald Trump as president, who promised repeatedly during his campaign that he would scrap the law.

Republicans have argued that Obamacare has led to rising premiums and a lack of choices in the healthcare market. Democrats say the purpose of the law was to lower healthcare costs, increase access and increase benefits for all Americans.

“If the ACA were to be repealed, tens of millions of Americans lose their coverage,” House Democratic Whip Steny Hoyer of Maryland said Monday during the press call. “And tens of millions of others will see their healthcare costs skyrocket.”

Hoyer bashed a plan that was recently floated by Republicans that would involve Congress passing a repeal plan, but delaying its effective date for at least a few years while a replacement plan can be developed.

“Republicans’ so-called ‘repeal and delay’ plan is code for ‘repeal without an alternative,’” Hoyer said.

Rep. Richard Neal of Massachusetts, top Democrat on the powerful tax-writing Ways and Means Committee, defended the law by arguing that it has led to 137 million Americans now having access to preventive services without cost-sharing, it has addressed chronic conditions like obesity and diabetes and it has expanded Medicaid eligibility.

The repeal bill Congress passed last year would have repealed the health-care law’s tax hikes, insurance exchange subsidies and the Medicaid expansion. The Congressional Budget Office (CBO) estimated that the plan would lead to 22 million people losing their healthcare insurance.

While Mr. Trump has made it clear he’s in favor of repealing the law, he suggested after the election that they wouldn’t dismantle the law without a replacement plan ready and would “do it simultaneously.”

“We’re not going to have, like, a two-day period and we’re not going to have a two-year period where there’s nothing,” he said on 60 Minutes when asked if people would lose their coverage.

Speaker Paul Ryan, R-Wisconsin, told 60 Minutes’ Scott Pelley in an interview in December that they want to repeal the law as soon as possible.

“We have to bring relief as fast as possible to people struggling under Obamacare,” said Ryan, who was then asked what it would be replaced with. “Patient-centered healthcare that gets everybody access to affordable healthcare coverage, so that they can buy whatever they want to buy.”

To cap off the first week of the new congressional session, Pelosi is calling on her Democratic colleagues to hold press events in their respective districts Saturday “to highlight the risks of repeal of the ACA and of ending the Medicare guarantee.” House Democrats are also coordinating a “Our First Stand: Save Health Care” day of action on Jan. 15 with their counterparts in the Senate including Chuck Schumer, D-New York, and Bernie Sanders, I-Vermont.

Watch: How Paul Ryan plans to replace Obamacare

Democratic governors are also voicing their concerns about a repeal. In a recent letter to Speaker Paul Ryan, R-Wisconsin and Senate Majority Leader Mitch McConnell, R-Kentucky, they argued that reversing the law would “throw millions of our residents” off their insurance coverage.

“Repealing the Affordable Care Act (ACA), including its Medicaid expansion, would be a financial and health disaster for states. Repeal would throw millions of our residents off their health coverage, shift enormous costs to state governments – blowing a hole in state budgets – and cause economic uncertainty for our states’ businesses, hospitals, and patients,” the letter said.

Without a replacement plan, repealing the law would risk making 30 million people uninsured, according to a study released last month from the nonpartisan Urban Institute. Of that total, it predicted 22.5 million would become uninsured because of the law’s subsidies being repealed and 7.3 million would become uninsured because of “the near collapse of the nongroup insurance market.”

Mr. Trump, for his part, has chosen a key Obamacare opponent as his nominee for secretary of Health and Human Services: Rep. Tom Price, R-Georgia, who was part of crafting previous legislation to undo the law.

Price is among eight people Mr. Trump has nominated to his Cabinet who Senate Democrats plan to target as congressional committees hold their confirmation hearings early this year, according to a report Monday by The Washington Post.

Pelosi, meanwhile, admitted Monday that if she could have done one thing differently on Obamacare, she said Democrats could have messaged it “in a much stronger way.”