It is, however, the "firmest" statistic, since U4-6 require things like surveys to estimate how many would be workers are "discouraged" through "underemployed". And you have to do that to distinguish them from e.g. voluntary stay at home mothers (and fathers).
Hard to say exactly. In addition to what jrbancel said about how the calculations differ, you can also refer to BLS to see which industries Americans are employed in:
http://www.bls.gov/emp/ep_table_201.htm
I would guess that construction, manufacturing, retail, and hospitality account for a significant portion of those jobs with less than 30 hours per week of work.
To me, it's fascinating that we have the presidential elections right?
Wouldn't someone who actually knows what to do and wants to fix things talk about these very things, in a very clear manner, hours on end, via something like youtube?
Is there a single candidate who has done that ever?
"Here are the numbers, here is what they mean, here is what we're going to do to change things around."
Is there someone I can look to to be better informed?
Well, I think there are a few things preventing this.
First, these kinds of big, complex problems are easy to talk about, but very difficult to provide a practical solution for. In this case, how do you get millions of Americans who are underemployed into better jobs without implementing something extremely expensive (like Universal Basic Income) or somehow boosting the economy in a meaningful way? You really can't, so anything a politician is likely to propose is probably going to be unfeasible.
Second, the average voter is really lazy when it comes to being informed about real issues and what can be done to solve them. 40% of voting-age Americans don't even bother to vote in presidential elections, let alone take the time to get informed about what problems the country is facing. And beyond that, most voters only want to hear what candidates will do about issues personal to them, which is why the candidates pitch their policies on as many things as possible shallowly. Chances are, if a person has a decent 40-hour a week job, they really don't care too much about the underemployment problems other people are having.
Third, the US unfortunately has a bit of an anti-intellectual problem that somewhat extends to elections. Voters just seem to prioritize likability and saying things they want to hear over intelligent discussion of complex issues. If things get too long-winded or complicated, most people just flip the channel to ESPN or whatever sitcom is playing.
I'm actually having trouble coming up with good sources for you that break down socioeconomic and political topics in an informative and unbiased way. CGP Grey on YouTube has a couple of good videos on some related topics, and TED hosts a number of talks on various issues, but I'm drawing a blank otherwise.
