Democratic presidential candidate has proposed a new 4 percent tax on the wealthiest Americans as part of her plan to finance new domestic spending initiatives. During a speech in Waterloo, Iowa, Clinton said the her "fair share surcharge" would apply to incomes above $5 million per year and raise an estimated $150 billion over 10 years. The campaign estimated the tax would apply to roughly 34,000 American households. The proposal is part of the former secretary of state's plan to implement the so-called Buffett Rule, a principle named for billionaire investor Warren Buffett that aims to ensure that wealthy Americans pay an effective tax rate of at least 30 percent.

Because of various deductions and shelters in the tax code, many do not currently pay that much — leading Buffett to lament that his secretary pays a higher tax rate than he does. Clinton, who faces competition from the left in Democratic primaries from Sen. Bernie Sanders, has proposed large new spending plans for such priorities as making college more affordable, expanding infrastructure, and bolstering access to child care and early childhood education. The Republican National Committee estimates the total cost of those proposals so far at $1.2 trillion over 10 years. Before she rolled out her new surcharge proposal Monday, the Committee for a Responsible Federal Budget estimated that Clinton's planned tax increases so far would raise slightly more than $800 billion over 10 years.

The surcharge plan would close slightly less than half of that $400 billion gap. Other elements of Clinton's tax plan haven't been made public yet. The Democratic front-runner has vowed to pay for her proposal without increasing taxes on Americans earning less than $250,000 a year. The campaign of Sanders, a self-described "democratic socialist" who has not yet specified his own tax plan, swiftly condemned Clinton's new plan as inadequate. "At a time of grotesque income and wealth inequality and when trillions of dollars have been transferred from the middle class to the top one-tenth of 1 percent over the last 30 years, Secretary Clinton's proposal is too little too late," Sanders spokesman Michael Briggs said in a statement.