Some arguments conclude by bedtime; some start in antiquity and go on and on.

In “The Rivalry,” a 1959 play by Norman Corwin now playing at the Irish Repertory Theatre in New York, a riveting old argument is at the center of the work, and the debate is as potent today as it ever was -- the argument over human liberty and the survival of the United States itself.

The year is 1859, and the two men having the argument are none other than Senator Stephen A. Douglas and his political rival Abraham Lincoln. Given the high stakes, and the banner names, there’s no question that Corwin’s morality play rewards the attention.

What is Democracy’s purpose, the play asks?  Is it to satisfy the desires of the majority? Or is it to impose the majority’s will on any minority that steps out of line?

Just ask the gay and lesbian citizens of California, still smarting from having the right to marry granted and then taken from them by a bare majority of their fellow citizens by Proposition 8, for an indication of how raw these arguments still can be.

In “The Rivalry,” the great debate is about the abolition of slavery -- which threatened to split the union -- and the questions the play asks can rattle us still.

If all men are equal, then how can a white man hold a black man in the bondage of slavery? If all men are equal, why are so many black men denied the right to vote?

The two men the playwright has chosen to mull over these questions are ideal. Douglas is an out and out racist. A man who speaks of the need to defend against the “mongrel races” of the world, a man who looks at black men as property rather than human beings.

Given the odiousness of his views, Corwin decides to introduce Adele Douglas, his much more genteel and beguiling wife, in an attempt to blunt the edges of her husband’s reprehensible views.

Douglas may well have been an adoring husband, but he was also a bigot, and his uncompromising views precipitated a national rupture that he first helped stoke before he eventually tried to stop it.

Having a beautiful wife who sits decorously on the stage while her racist husband bellows at the crowd is no real distraction, in the end. The playwright is asking us to observe the humanity of a man who refused to do likewise for every African American he ever met.

What would Douglas -- or Lincoln, for that matter -- have felt about the possibility of a black man becoming president a century and a half later? This tantalizing question arises as the play progresses.

Other interesting questions arise too about the nature of justice and the era in which it is handed down. Corwin reminds us of the infamous Dred Scott case of 1857 (in which the then Supreme Court ruled that a black man was in fact no more than property). Nowadays that ruling appalls us, but in 1857 it was accepted as fact by half the nation.

As Lincoln, Christian Kauffmann is perfectly cast as the melancholic national leader, managing to convey the private cost of a bloody philosophical facedown. After a tremulous start, his characterization deepens to the point where he almost conjures the tall, solitary and cerebral man he’s playing.

Awarding winning Irish director Vincent Dowling, the former artistic director of Dublin’s Abbey Theatre, gives the production an ideal pace, letting the play’s themes emerge through each nuanced performance. Dowling, an Emmy winner, has a formidable theatrical background, and Corwin’s drama plays to every one of his strengths.

Peter Cormican, who plays the so-called “Little Giant” Douglas, does well with a deeply unsympathetic character, allowing all sides of the man to emerge. 

Mary Linda Rapelye as his wife is particularly heartbreaking as she slowly awakens to the injustice she has been a part of.

“If God gave has given the black man but little, that little let him enjoy,” says Lincoln, in a simple, heartfelt plea to the nation’s better angels to live and let live.

“The Rivalry” is now playing at the Irish Repertory Theatre, 132 West 22nd Street, until July 19. For tickets call 212-727-2737.