She's supposed to be a 2020 presidential juggernaut, but lately U.S. Sen. Elizabeth Warren's campaign is stumbling along like a clunker.Warren blundered her way through what should have been a softball interview with CNBC, raising the possibility she wanted to raise the top tax rate to 50 percent or higher.Here's how I look at budgets and taxes, Warren lectured. A lot of people think they're just numbers, they're not. They are the expression of our values.