Is “have the steel” an idiom in the statement, “Mitt Romney would have the steel to order the 2011 operation that killed Osama bin Laden in Pakistan”?

Based on the context of the article, it seems pretty clear that "have the steel" refers to having bravery, fortitude, resolve, and the ability to "not blink under pressure."

I would suggest that the term "have the steel" is not yet an established idiom (as your research indicates), but it may be one writer's shorthand for "have the steely resolve," as "steely resolve" is an idiom that has been has been growing steadily in popularity over the past 20 - 30 years. Perhaps this author thought it was time to shorten that phrase, lest it come across as trite.

To answer your last question, can you jump on board, and say, “He had the steel to tell his boss ‘you’re wrong’?” – I think you can. I can't imagine being confused by that wording. The word "steel" has been used often enough similar contexts (such as "nerves of steel"); I think it's clear what you mean, and the message is unambiguous.

I'd never head of someone 'having the steel' so (based on a tiny sample size admittedly) I don't think it's all that common. Again - tiny sample size, grain of salt...

As to what the writer was going for... well, my opinion is the original author was trying to evoke something along the lines of 'would Mitt Rommney have had the willpower and determination to order the operation and see it through?' much like @fumblefingers said.

I don't know if it'd necessarily be usable as "He had the steel to tell his boss ‘you’re wrong’?" as I don't get enough of a sense of making a considered decision and being both aware of the long term work required to enact it.

Just as a fair warning, text below this point is me trying to unpack in my head why I've got this particular translation of 'to have the steel to', and so warnings as to whether or not it makes complete sense should apply.

I come to this point based on pretty much the same thought process @fumblefingers describes, except my particular experiences with the entire iron vs steel comparison (in both literal and literary senses) don't go with durability in a sense of corrosion and wear, but more in terms of how the two materials resist force, and ultimately, a question of endurance.

Specifically, the distinction is that both are strong but where iron will tend to fail catastrophically and shatter, steel will tend to bend and deform, yet remain roughly true to the original piece. So on one hand, something incredibly strong and unchanging, but still where if you could muster enough force, it might break, versus something that you almost certainly can deform, but could never truly distort from it's original form, and is thus, ultimately enduring (even if it's going to get a bit beat out of shape).

• This sort of explains (to me) why you get someone with an iron will, and a ruler with an iron fist - both phrasings going for the entire 'unshakeable, unchangeable, unstoppable, unyielding, uncompromising' type of feeling. Probably not related, but an iron maiden also fits into this category.

• On the flipside, you then have someone described as having nerves of steel (someone who in the face of incredible adversity is going to be stressed out, and might waver, but is absolutely never, ever, going to break) or suchlike. One contemporary reference that comes to mind is a tweet from cyclist Robbie McEwen: "...half of stage simple in comparison.Aussie steel- bends but doesn't break ;)" in reference to a particularly bad day on the 2010 Tour De France where he spent most of the last half riding alone, up some awful, awful mountains. Normally the sprinters like him would form a group and grind up the mountain together to save energy and well, actually make it to the end. Usually a lone sprinter when faced with something like that would actually just drop out of the race and retire.. McEwen in this case is referring to being particularly bloody minded, and determined to finish the stage hence the entire steel/endurance thing coming out.

• I've also read in more than one place (but can't place it, sorry) where an individual's faith has been likened to steel in a positive sense in that the individual has the capacity to shift and adapt, but is fundamentally enduring. This was contrasted with someone who was overly orthodox in their faith and being likened to iron, funnily enough. One of the unspoken things in the story being the comparison between a 'pure' (read:naive) form of faith, vs a more practical (read:tempered) form of it. Could've been a novel, seems like the kind of thing you'd get as a lesson-teaching type of thing.

• lastly, and this is just purely physical - steel is arguably a more refined form of iron, with precisely controlled amounts of impurities, and as such, is a bit more 'refined' or 'advanced'

So, with all of that bouncing in my head, and going back to the original writer, while I don't think it was intentional, you could actually read it quite poetically if you take steel as meaning 'strong, but flexible, and enduring, and a more refined thing':

...the question of whether Mitt Romney would have been strong enough and wise enough to swallow his own personal disgust at ordering a man be killed, knowing full well the kind of criticism it would (possibly rightfully) attract, and whether or not he would be mature enough to calmly weigh the alternatives, make the decision, and see it through without waver from it if he truly believed it was the best thing to order the 2011 operation that killed Osama bin Laden in Pakistan.

OED gives as one of the figurative senses for steel (specifically in the domain of sport) - Power of endurance or sustained effort. I think the allusion stems from the fact that steel lasts longer (corrodes less) than iron, which is more often figuratively referenced for its hardness (iron fist, The Iron Lady, etc.). Personally, I think it's a somewhat dated expression.

Thus to have the steel [to do something] doesn't really mean have the nerve. It means have the staying power to see something through that might involve protracted effort. But in practice, because the expression is quite rare relative to nerves of steel, very likely many people will understand them to mean much the same thing.

I have to say it seems quite possible to me the writer in OP's case may have conflated the two idioms. It's not obvious that Obama needed a lot of "endurance" to order the raid - it was something which would either succeed or not, in a fairly short timeframe. One might more appropriately ask whether a leader had the steel to engage militarily in Afghanistan, since by now I'm sure everyone knows that's something that really is expected to take a long time.

I don't think it is, or needs to be, an idiom.

steel = resolve