This little phrase appears to be the backbone of the liberals who defend Obamacare as constitutional. Lo and behold, a quote from Thomas Jefferson, from Opinion on National Bank in 1791 wrote this:
"They are not to do anything they please to provide for the general welfare, but only to lay taxes for that purpose. To consider the latter phrase not as describing the purpose of the first, but as giving a distinct and independent power to do any act they please which may be good for the Union, would render all the preceding and subsequent enumerations of power completely useless. It would reduce the whole instrument to a single phrase, that of instituting a Congress with power to do whatever would be for the good of the United States; and as they would be the sole judges of the good or evil, it would be also a power to do whatever evil they please."
So when you hear or are confronted by someone using the "promote general welfare" argument, you can answer with what the thinking was then and is logical today.