Sen. Bernie Sanders, I-Vt., who is running for president and has greatly influenced Democrats' policy positions, said Tuesday that he does not support a bill introduced by House leaders to expand Obamacare.

"No, I support the 'Medicare for all' single-payer program," Sanders said when MSNBC's Chris Hayes asked him about the legislation that was introduced earlier that day. Asked a second time to clarify his stance and whether he supported incremental changes to the healthcare system, Sanders again replied that he did not support the bill.

Sanders, who was brought on to support Obamacare nine years ago even though he wanted more liberal changes on healthcare, had previously co-sponsored a bill that was similar to the new Democratic plan that he now opposes. He also previously co-sponsored a bill that would let people buy into Medicaid as an alternative to private health insurance.

But he also has been the leading voice behind supporting a healthcare system fully financed by the government, which would dismantle private health insurance in favor of a public plan. Few others used to share Sanders' position, but it has now become mainstream among Democratic presidential candidates. In the House, at least 107 members have co-sponsored the Medicare for All Act, which would expand Medicare to cover more services than it does now.

[Related: 'Medicare for All' loses Democratic cosponsors]

Despite a large portion of the caucus favoring this overhaul, Democratic leaders in the House have pressed forward with an Obamacare expansion measure. They have framed their bill as necessary in light of Trump's Department of Justice asking an appeals court on Monday to kill the entire law.

Tuesday's bill, known as the Protecting Pre-Existing Conditions and Making Healthcare More Affordable Act, would funnel more money into Obamacare and undo actions by Trump on Obamacare that Democrats view as damaging.

When asked whether he would vote against the Obamacare expansion bill if it were to reach the Senate, Sanders didn't directly answer but said the country should work toward the Medicare for All Act.

"Right now we're working on what I have fought for my entire life," he said. "Healthcare is a right. It has to be comprehensive. The current system is dysfunctional. It is enormously wasteful."

Sanders did point out that another bill, called the Medicare for America Act that lets people enroll in Medicare as an option rather than obligating them to do so, did not stand a chance in receiving a vote in the Senate, where Republicans have control. GOP Senate leaders have said the same of the bill to expand Obamacare.

Still, House Democrats are moving swiftly to advance the Obamacare bill so they can signal they will be prepared to address healthcare should Democrats gain control of government. Getting the support of liberal wings of the party, where Sanders is influential, will be key to securing the bill's passage.

Sanders' office did not immediately reply to a request for more information about his stance on the bill. Shortly after the MSNBC interview, he tweeted: "We must defend the [Affordable Care Act] from Trump’s assault and protect people's existing coverage. However, protecting the ACA will not fully solve the healthcare crisis. To finally guarantee healthcare as a right, we must take on the insurance industry and pass a Medicare for All bill."

We must defend the ACA from Trump’s assault and protect people's existing coverage. However, protecting the ACA will not fully solve the health care crisis. To finally guarantee health care as a right, we must take on the insurance industry and pass a Medicare for All bill. — Bernie Sanders (@BernieSanders) March 27, 2019

[Opinion: Here's how Bernie Sanders' 'Medicare for all' proposal would outlaw private healthcare plans]