(CNN) When Sen. Bernie Sanders did this in 2013, he did it alone.

On Wednesday, nearly four years later, Sanders introduced a new "Medicare for all" health care bill with a third of the Senate Democratic caucus by his side.

Flanked at first by New York Sen. Kirsten Gillibrand and Connecticut Sen. Richard Blumenthal, Sanders called the costs of the current system "insane and unaffordable," promising that the average family would benefit financially under his plan "because you will no longer be writing checks to private insurance companies."

For those whose taxes would go up, he added, "that expense will be more than offset by the money are you are saving with the elimination of private insurance costs."

Sanders said he planned to take the bill on the road to "every state in the country and hear what the people have to say" for a roving national workshop. But the outreach came with a warning. He fired a shot across the bow of Republicans likely to oppose the bill, saying the GOP had "no credibility on the issue of health care."

"To my Republican colleagues, please don't lecture us on health care," Sanders roared to applause from the dozens of activists in the room. "In the last few months, you, the Republican Party have shown the American people what you stand for" by largely voting to dismantle Obamacare.

Taking her turn, Massachusetts Sen. Elizabeth Warren framed the push for single payer as the latest step in a long liberal project to expand access to medical care -- from Franklin Roosevelt's social security to Medicare and Medicaid under Lyndon Johnson and, most recently, Barack Obama's signature legislation.

"But we are here today to take another step," she said. "We will not back down in our protection of the Affordable Care Act, we will defend it at every turn, but we will go further -- we will go further and we will say that in this country, everyone gets the right to basic health care."

Before Sanders' remarks, supporters shared personal, often visceral accounts of doctors currently struggling to provide care and patients in need. An oncology nurse drew gasps in the room as she recounted the story of a cancer patient who could not afford to continuously empty the fluid building up in her lungs. The price of drainage bags, she said, was only covered in part by insurance, forcing the woman to ration her use.

The legislation unveiled on Wednesday afternoon in Washington arrived a little more than six weeks after the party's all-hands fight to preserve Obamacare secured a narrow reprieve on the Senate floor . Sanders' plan, though dead-on-arrival in a Republican-controlled Congress, offers a blueprint for fundamentally reshaping the American health care system by moving the country to a government-run, single-payer program.

Under Sanders' proposal, Americans would receive a "Universal Medicare card" that would be a ticket to comprehensive health care services, including hospital stays, doctor visits, substance abuse treatment, dental, vision and reproductive care -- including abortion.

However, consumers may have to pay up to $250 out-of-pocket for prescription drugs, with incentives to use generic medications. Sanders, who's been vocal about lowering drug prices, would allow the government to negotiate with pharmaceutical companies.

Long-term care, which Sanders had included in the version he unveiled during his presidential campaign last year, will not be covered. It will be addressed in separate legislation, an adviser to Sanders told CNN.

The bill calls for the elimination of premiums for private health insurance, deductibles and co-pays. However, most Americans and businesses will pay what Sanders says is a much smaller share of their income to fund the program.

While the new legislation has not yet been scored, the program Sanders pitched on the campaign trail came with an estimated annual price tag of nearly $1.4 trillion, to be paid for in part by a proposed new 2.2% income tax on all Americans, a 6.2% levy on employers and a further round of tax hikes on the wealthy.

California Sen. Kamala Harris made the business-friendly case for single-payer. (Or, as one leftist activist in the room grumbled afterward, "the neoliberal case.")

"Let's give the taxpayer of the United States a better return on their investment," she said. "Why? Because 'Medicare for all' stands for the proposition that all Americans from the day of birth, throughout their lives, will have access to health care."

The talk on Wednesday was broad, with little mention of the financial mechanics needed to propel such a massive program. But Sanders' plan does address one of the stickier questions facing such proposals: how to phase out the current system, which depends primarily on employer-sponsored private plans, without creating a disruption in care.

The answer, according to this new version of the bill, is to roll it out over a four-year transition period, a process Gillibrand and her office had a big hand in crafting.

The law would beef up Medicare -- adding dental, vision and hearing aid coverage -- in its first year, while reducing the eligibility age to 55. Children, up to age 18, would also be offered immediate access to the government-run plan. Over the next two years, the Medicare age would drop to 45, then 35 years old. By the fourth year, everyone would be eligible.

Despite the considerable legislative obstacles in its way, single-payer as a concept is gaining traction among the public. Some 53% of Americans support a national health care plan , according to a June poll by the Kaiser Family Foundation. That's up from 50% last year and from 40% between 1998 and 2000.

It is also notably increasing in popularity with some of the most ambitious Democratic officeholders -- a group of Senators possibly looking to run for the party's presidential nomination in 2020.

Harris was the first to announce plans to cosponsor the bill, followed by Warren. New Jersey Sen. Cory Booker and Gillibrand made their plans public on Monday and Tuesday. The names, some expected, others less so, rolled in right up to a few hours before the event, when New Hampshire Sen. Jeanne Shaheen announced her support.

Notable absences, however, included Senate Minority Leader Chuck Schumer of New York, Ohio Sen. Sherrod Brown, California Sen. Dianne Feinstein and the party's 2016 vice presidential nominee, Virginia Sen. Tim Kaine. Of the latter three, all up for reelection in 2018, only Brown is expected to face a serious Republican challenge.

Single-payer activists, mostly from the progressive grassroots movement that fueled Sanders' 2016 presidential campaign, are delighting in a sense of momentum after years stuck on the margins of the debate.

"This is an especially gratifying moment for the tens of thousands of nurses across the US who have dedicated years of effort to transform our health care system from an profiteering industry based on greed and suffering to patient need and healing," National Nurses United Executive Director RoseAnn DeMoro said in a statement.

She called the bill's introduction a "direct product of how Sen. Sanders made Medicare for all, and healthcare as a human right, such a signature issue of his campaign."