Conference submission while article under review for journal My co-authors and myself have recently submitted an article (in the wider field of economics) to a journal and we now wish to submit the same article to a conference while the article is still under review at the journal. There will be no such thing as published conference proceedings and as far as I can tell not even a working paper version of the article will be made available online. 

Is there anything unethical about doing so, i.e. submitting an article to a conference while it is under review for a journal?

I am asking since for a journal submission one would write in the cover letter something along We confirm that the article has not been published or is under review elsewhere. Now I am drafting the cover email to the conference submission and I was wondering whether I should include a similar phrase and if so what to write?

For a conference submission, should (must/can) I indicate that the submitted article is currently under review in a journal, or would I not want to write any such sentence?

Currently I would not, since as long as the article has not been accepted it is still work-in-progress and as such perfect to submit at a conference (economics conferences also tend to attract fairly advanced-stage papers in my perception). 

I have asked my co-authors, who are experienced and established professors in the field, on the first question and they both saw no problem. I am asking for a second opinion to calm my mind since I am likely over-thinking the issue and unsure how to address it.
I have found one related question:
Simultaneous submission short version to conference and full version to journal
but in that field it is common to have a conference paper first, often followed by a journal article. As I stressed above, the difference here is that there is no conference publication as well as the very different fields.
Submitting journal version of an article while conference version is still in review
asks about the other way round, submitting to a journal while the conference article is still under review. Here is different in that the article submitted is the same, there is no conference publication and the order is the other way round.
In economics it is entirely normal to submit a paper to a conference while it is under review at a journal. I write as an experienced academic economist who has done this (and seen it done) many times over the past 15 years. So it is fine for you to submit to the conference. No disclaimer/declaration is necessary.

The economics discipline has developed very permissive norms about this kind of thing out of necessity because acceptance rates at even mediocre journals are low (typically not much greater than 10%) and review times long. It's not unusual for a delay of 3 years to elapse between first submission to a journal and eventual acceptance at a (likely different) journal. Moreover, economics papers often take a long time to write and, as you correctly intuit, the most common practice is for economists to submit a paper to a conference only once a somewhat complete working paper is ready. If we were to insist on a strict sequencing of tasks then then result would be a decade-long cycle for a paper, whereas allowing overlap between writing, conference presentation, and submission allows work to be developed, circulated, and published somewhat more quickly.

All that said, you might like to consider why it is that you are submitting your work to the conference. An important reason, of course, is to disseminate your ideas. But another key purpose of conferences is that they provide an opportunity to obtain feedback that can improve your work. This is especially valuable the first few times you present, when you will quickly get an idea of what the controversial assumptions in your paper are, and what people perceive as the main weaknesses. It is often helpful to delay journal submission until at least after you have presented the paper a couple of times so you can figure out what issues the referees are most likely to raise and address these in advance. Given that you probably have to wait 4-6 months to get a set of referee reports back from a journal, it is often much more efficient to first get feedback on the conference circuit.