Sen. Elizabeth Warren (D-Mass.) called on her fellow Democrats on Thursday to "go on the offense" to improve the country's healthcare system instead of focusing on simply defending the Affordable Care Act (ACA), also known as Obamacare.

Speaking at a conference for the pro-Obamacare group Families USA, the senator noted that while the ACA has been credited with insuring 20 million Americans who previously had no health coverage, 28 million people remain uninsured nearly eight years after the law was passed.

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"We need to build on that progress and do more to hold America's insurance companies accountable," said Warren, who is considered a potential challenger to President Donald Trump in the 2020 election.

The senator also touted her support for Sen. Bernie Sanders's Medicare for All proposal, saying it provides a plan "to give every single person in this country a guarantee of high-quality coverage."

"This is a goal worth fighting for and I am in this fight all the way," she added. "For too long giant insurance companies have pretty much run the show."

Since the ACA passed in 2010, insurance companies have restricted the doctors patients are able to see, raised premiums, and dropped coverage for certain prescriptions with no warning.

Many insurance companies left the state-run health insurance exchanges in 2017, complaining of financial losses as they were covering people with more health issues than they had before the ACA, and as Trump ended cost-sharing payments for coverage of low-income households.

When companies leave, said the senator, Congress should "call their bluff" and "replace their policies with public alternatives."

"Private insurance companies are failing the American people," Warren said. "There is no reason on earth for us to continue to allow the healthcare of the American people to be held hostage by an industry that both attacks any new healthcare proposals and at the same time refuses to do anything to fix it."