Why is that a gaffe?
Well, to start with—as NBC's Mike O'Brien points out—Mitt Romney earlier this month said that he wouldn't celebrate anything less than four percent unemployment. So in the span of less than a month, Mitt Romney has already moved his unemployment rate target up by two full points—a 50 percent increase.
Second, the nonpartisan Congressional Budget Office already projects that under current policy the unemployment rate will fall to six percent by the end of 2016. In other words, Mitt Romney's pledge is no better than current projections.
Finally, President Obama's economic team has already projected a six percent unemployment rate in 2016. So Mitt Romney isn't promising to do any better than the president's own projections.
But it's even worse than that for Romney. The White House projections were developed in the late summer of 2011 using very conservative growth assumptions. For example, they projected a fourth quarter 2012 unemployment rate of 8.8 percent, but it's already down to 8.1 percent. As a result, in February the White House said that they expected unemployment would be even lower than they had originally projected ... which means Mitt Romney is pledging higher unemployment rate than President Obama. And if that's not a gaffe, I don't know what is.