There is a lot packed into this, so I'm going to try to get a quick overview down and then we can deal with more details in follow-ups if necessary.

It's possible someone did, but I'm not aware of any founder using the phrase. The general concept is often attributed to them, usually through Jefferson's metaphor of enslaving being like having the wolf by the ear: you can't let go or you get bit but also holding on is a good way to get bit and also to make the wolf want to bite you.

Calling enslaving a necessarily evil is usually done in the context that it is unfortunate in some ways, and the speaker regrets them. That may even be true, though American founders get graded on so generous a curve that even the most useless and vague condemnations of enslaving are treated as spectacular moral victories. Actually arguing to keep people enslaved is often insufficient to disqualify a founder from being deemed antislavery, a situation so absurd that to state the argument is to refute it.

Yet here we are. So what's it mean to say that slavery is a necessary evil?

First off, it means one is in favor of slavery. Whether we're talking specifically about Jefferson's metaphor or a more generalized appeal to worse consequences, the necessary evil argument is that enslaving people may be unfortunate in some abstract sense, but not enslaving them is worse. To all of this we must add that it is far worse for the enslavers. They, not the enslaved, would risk loss of property, wealth, and lifestyle that they stole from the bodies of the people they enslaved. As people who closely equate their property and lifestyles with their actual persons and worth as human beings, this is a horror almost beyond comprehending for most American enslavers. The kicker, of course, was that these were tremendously important to them and also still less important -or so one hopes- than the safety and lives of themselves and their loved ones.

Enslavers knew better than anyone except the enslaved themselves just what went into keeping people enslaved. That goes double when the enslaved outnumber the whites in the area by a large proportion -a common situation in the cotton states, which include Cotton's Arkansas- and call it triple for the common case in such places that they also outnumber the whites present in the household. Slavery is violence, torture, and rape used as a system of labor and social organization. Enslavers know that and also know what they would do if such a thing were done to them and theirs. If the enslaved were freed, then must it not mean that they would turn the tables at once? A genocidal race war must then prevail. Preventing that imagined cataclysm, on top of the other stuff, makes slavery necessary so far as the necessary evil argument goes.

Where does that leave us? The enslaved are always subjected to treatment which white Americans understand that they should feel free to answer with violence should it be done unto them. Emancipation will always involve a loss of wealth and property because enslaved lives are both, and also the engines by which more of each can be acquired. They must, therefore, remain enslaved forever. At no time does the imagined necessity vanish -it cannot while slavery exists- and therefore enslaving must go on until the end of time. If there is no circumstance under which one will countenance a plan of emancipation, one cannot even in the most minimal sense be construed as anything more than a staunch defender of enslaving people.

So yes, Thomas Jefferson and company are proslavery theorists. They are committed, first and foremost, to the preservation of slavery and were often enough prepared to scuttle the Union itself should enslaving be materially threatened. -No one took them up on the bet, but it was credible enough to get substantial concessions at Philadelphia. These in turn act to preserve slavery, not slowly strange it.- Their counterparts outside the major enslaving states can do much, much better but few of them go around calling enslaving in any respect necessary. It's, to them, just an evil and one which some of them devoted considerable energy to removing nearer to home. It's still not an evil for which they're prepared to sunder the Union and so they are still prepared to countenance unspeakable horrors to preserve their national project, which cannot be forgotten or minimized...but they do a damned sight better than a few Virginian blowhards for all that the bar is low enough to be buried in the center of the earth.

To stop here we must of course exclude the founders from lesser enslaving states, which had successful -if often quite slow and still controversial to whites- emancipation projects. We must also exclude the great many white Americans, founder and otherwise, who enslaved black Americans happily. The spectrum of thought, even just confined to white elites, does not run from "slavery is terrible and end it immediately" to "slavery is awful but what can you do except keep it forever?". The latter include most of the people who get talked about in this context, which says a great deal about traditional American discourse about slavery and none of it is remotely good. We must go deeper, as humans are habitual innovators in goods and evils.

South of Virginia, and even north of it and below Pennsylvania, save for a brief wave of emancipations after the Revolution which still constitute only a small minority of enslavers, finely-calibrated frowns about enslaving people drop off a cliff. The spectrum of white thought does not stop at people who make those choreographed frettings with whip in hand. We must also consider the enthusiasts, who believe enslaving people is right, moral, and good on its own sake. These people believe, as many of the necessary evil types also do, that an enslaving society is better than one based on free labor. Enslaving is helpful, even necessary, to cultivate proper civic virtues and demonstrate one's manhood. To be a person in good moral standing, to them, virtually requires being an enslaver. This reaches its formal heights in South Carolina's constitutional requirement that members of the legislature enslave a certain number of people to qualify for office, but goes far beyond that.

One may argue from a very selective and out of context reading of the documents, and the relevant histories, that these people somehow put slavery on a path to ultimate extinction. Lincoln and his Republicans made that argument. It was a useful one to make, politically, and I expect many of them believed it to be so. They were wrong. It is true that some founders, some of the time, think slavery will go away on its own. It is also true that many have no interest in that ending and would fight to halt its progression. They will scream bloody murder at even the thought of slavery's spread being limited in anywhere they expect it can realistically go. But if we take a hard look at the actions of the founders, evidence for such a conviction is, at best, insubstantial.

The founding generation can imagine a world without enslaving; they need only look to the north. They can imagine peaceful transitions to freedom and then enact them. They can also reject the same and insist not only that slavery must remain forever where it already exists but must be permitted to expand to new places where it will also remain forever, preserved against all hazards. To argue that "the founders" agreed on anything except, maybe, the wisdom of rebellion in 1776, is a tremendous reach. To argue they were of one mind about enslaving is to write fiction. Many don't care at all. Many are keen to step up enslaving. Those two groups effectively control the nation's government and direct its policy without significant challenge until the 1850s and without effective challenge until the states most committed to enslaving people, like Cotton's Arkansas, have withdrawn their representation in Congress by means of treason in defense of slavery.

All of this goes to say that historians ought to commend the Senator on the accuracy of his pronoun use. Should this quest for knowledge imperil his notion that the United States is and was, as he says in the article "an imperfect and flawed land, but the greatest and noblest country in the history of mankind” that does not make it bad history. If patriotic myth cannot withstand historical investigation, then it is not the historian's task to preserve the myth.