(CNN) Sen. Elizabeth Warren is proposing a new tax on Americans with assets of $50 million or more as part of a bold new initiative to reverse the increasing concentration of wealth among the country's richest households.

According to documents provided by Warren's presidential campaign, the plan would impose a 2% tax on Americans whose net worth exceeds $50 million, with an additional 1% levy on billionaires. It does not specifically address marginal tax rates.

The proposal is among the first and most detailed steps taken by a top-tier Democratic primary hopeful to address what has become, in the aftermath of the 2008 financial crisis and Occupy Wall Street movement, a key issue for the party as a leftward shift that began during the 2016 primary accelerates under President Donald Trump.

Taxing wealth, however, could be very difficult to do. First, there are questions about whether it is permitted by the US Constitution, with legal scholars falling on both sides of the argument. The Constitution gives Congress the power to levy taxes -- such as tariffs on commerce. However, it places limitations on so-called "direct taxes," though the definition of these has long been unclear.

A 19th century Supreme Court ruling struck down a federal income tax that existed at that time. But the 16th Amendment, which was ratified in 1913, established Congress' right to impose a federal income tax. Also, an early 20th century Supreme Court ruling upheld the ability of Congress to levy an estate tax.

Read More