Two of the most powerful economic voices in America disagree on a tax for the uber-rich

Democratic Senator Elizabeth Warren faces an uphill battle persuading Washington to back her tax on ultra-millionaires — starting with convincing her own party's Treasury Secretary.

Janet Yellen, whom Warren praised last year as an "outstanding choice" to lead the Treasury Department, fears a wealth tax would be too messy to implement. "Then Janet and I need to have a conversation about implementation," Warren told CNN Business on Monday after introducing her Ultra-Millionaire Tax Act.