Hillary Clinton just proposed a new tax on the richest Americans

Former Secretary of State Hillary Clinton's presidential campaign released a new tax proposal on Monday that it called the "Fair Share Surcharge."

An aide to the Democratic front-runner said the plan was for a 4% "surcharge" on Americans who make $5 million or more annually.

The aide argued that the tax was justified because many of the richest Americans pay a lower effective tax rate than people who make far less money.

According to the aide, Clinton's plan goes further than the so-called Buffett Rule, the Warren Buffett-inspired tax plan that proposed a minimum 30% tax on incomes above $1 million annually. Clinton has embraced that proposal on the campaign trail.

The Clinton campaign aide said the tax would raise an estimated $150 billion over 10 years.

The proposal comes as Clinton's campaign approaches the home stretch in the first two voting states, which weigh in at the start of February. In recent days, Clinton surrogates have increasingly questioned the liberal credentials of her chief rival, Sen. Bernie Sanders (I-Vermont), on issues like gun control and Wall Street reform.

The Clinton campaign said it would soon release additional proposals to make the wealthy pay a fairer amount into the system. She's also apparently set to announce tax-relief proposals for the middle class.