U.S. Democratic presidential candidate Hillary Clinton on Monday proposed a 4 percent tax on the wealthiest sliver of taxpayers who earn more than $5 million per year.

The so-called "surcharge" on the wealthiest 0.02 percent would generate $150 billion over the next decade, according to a Clinton campaign aide.

The suggested tax follows Clinton's promise last month as she campaigned alongside billionaire investor Warren Buffett to build on the "Buffett rule," which would establish a minimum tax rate of 30 percent on those earning more than $1 million per year. Buffett has criticized tax policies that allow the rich to pay lower rates than the middle class.

"I want to go further and impose what I call a fair share surcharge on multi-millionaires because right now, we’re behind and we need to get the wealthy and the corporations to pay for their fair share, so I can keep my promise, which is I will not raise taxes on the middle class," Clinton said at a campaign stop in Iowa on Monday.

The Democratic front-runner, Clinton is just weeks out from the first party-nominating contests in Iowa and New Hampshire, where she is trying to fend off her chief rival, U.S. Sen. Bernie Sanders of Vermont, a democratic socialist who has made reducing income inequality the theme of his campaign.

Clinton's proposals have included a $350 billion plan to reduce college debt; a $275 billion plan to invest in infrastructure and a $30 billion plan to assist coal-dependent regions as the country transitions to renewable energy.

At the same time, she has promised not to raise taxes on families earning less than $250,000 per year, and whose support she needs to secure the Democratic nomination ahead of the election in November.

Clinton will release more tax proposals later this week that were "designed to ensure the wealthy pay their fair share," her campaign said.

For more on the 2016 presidential race, see the Reuters blog, “Tales from the Trail” (here).

(Reporting by Amanda Becker; additional reporting by Alana Wise; additional; writing byDavid Alexander; Editing by Eric Walsh and Bernadette Baum and Alistair Bell)