Sen. Bernie Sanders, I-Vt., on Wednesday declared he was gearing up for a fight against pharmaceutical and health insurance companies as he works to gain support for his single payer healthcare bill, and brushed off the Republican opposition to his proposal that quickly emerged.

Sanders, who did not take questions from reporters following a press conference on his bill, the Medicare for All Act, slammed the current healthcare system as one that "allows insurance companies and drug companies to make billions of dollars in profits and make industry CEOs extremely wealthy."

He invoked the battle in the Senate over a healthcare bill to repeal and replace portions of Obamacare, which most Republicans voted for even though it ultimately failed in July.

"To my Republican colleagues: Please don't lecture us on healthcare," he said. "In the last few months you, the Republican Party, have showed the American people what you stand for when you voted for legislation that would have thrown up to 32 million people off their health insurance they have and give huge tax breaks."

"You have no credibility on the issue of healthcare," he added.

In response to the proposal, Senate Majority Leader Mitch McConnell, R-Ky., tweeted that, "The Left is demanding an actual government takeover of healthcare," and shared a link to a list of stories that stated the single-payer proposal was a "litmus test" for Democrats.

Sen. John Barrasso, R-Wyo., chairman of the Republican Policy Committee, used similar language to describe the bill, and said outside the Senate floor, "While Bernie Sanders' slogan may be popular, it's really the nuts and bolts and details that matter the most, and that's in terms of money, in terms of time and in terms of their freedom of choice when it comes to healthcare."

At the White House, President Trump spokeswoman Sarah Sanders said she couldn't "think of anything worse than having government more involved in healthcare." She noted that Sanders, who ran against Hillary Clinton for the Democratic nomination for president, lost the nomination, which she said signalled the American people do not support a single payer option.

A poll from the Kaiser Family Foundation found that 53 percent of respondents say they favor a single-payer system, but support falls off when arguments are made about the government having more involvement in healthcare and when questions are asked about financing through higher taxes.

The bill will not be able to pass in a Republican-controlled Congress, and is opposed by several Democrats, including House leaders. Still, it has an unprecedented backing from 16 Democratic senators, including possible 2020 presidential hopefuls Democratic Sens. Cory Booker of New Jersey, Kamala Harris of California, Elizabeth Warren of Massachusetts and Kirsten Gillibrand of New York. A majority of lawmakers in the House have co-sponsored a similar bill.

Under the Sanders healthcare plan, Medicare would pay for emergency surgery, prescription drugs, mental healthcare, and eye care without a copay. Medicare, which does not currently cover dental care or vision and hearing aids, would cover those items under his proposal. Soon after passed into law, people 18 and under would receive a "universal Medicare card" and others currently not eligible for Medicare would be gradually phased in after four years.

People who now receive private medical coverage under a job would lose that plan to receive Medicare instead, and their employers would pay higher taxes rather than pay for the cost of private plans.

Sanders has not said how the bill would be financed, though he made a paper public that raised several possibilities, including through taxes on households or employers. During the press conference he said the plan was for he and other Democratic senators to travel the country to take input from the public and also from hospitals. A public option, he said, would be "more than offset by the money you are saving by the elimination of private health insurance costs."

An analysis of a similar plan Sanders introduced when he was running for the Democratic nomination for president, from the left-leaning Urban Institute, projected that such a system would increase federal spending by $32 trillion over a decade. With more costs shifting to the federal government, the analysis projected that the private sector would spend $22 trillion less than it otherwise would have and states would spend $4 trillion less.