I had a professor in law school who drilled his students to begin every answer to a legal question with “it depends.” He’d stop us if we forgot to begin our answer that way, smile encouragingly as one of us began with this phrase and wave his hand for the follow up, an explanation of the devilish details on which your actual answer depends. He was no radical, as far as I know (he taught Intellectual Property, the area he also practiced), but that qualification, “it depends”, turns out to be the key to all critical discussion of our laws.
If you are eighteen and your girlfriend is sixteen, in many states you can be easily convicted of statutory rape. Will every eighteen year-old sleeping with his sixteen year-old girl friend be prosecuted for this crime? Obviously not, but why not? It depends. On who the boy is, who the girl is, who their families are. A prep school kid from a rich family can be excused for letting his heart rule him that way. It seems so unfair to let this youthful love affair (even — yea, especially– if it is only one night long) destroy the boy’s entire life! A hard boy from the ghetto will generally get a less generous pass from the law for letting his heart decide what the law should be. Additionally, justice reasons, the kid from the ghetto should know exactly how ungenerous his pass will be. Impossible to compare the lives of those two boys, one a hard criminal type, the other the finest sort our society produces!
Is contempt of a subpoena to testify a crime? It depends.
The Attorney General is instructing the president on a (very weak) “protective presidential privilege” defense for his ongoing obstruction of justice. He has given Trump’s energetic attempts to cover up all potentially damaging information against him the tiniest, most obscene fig leaf of legal cover. On the plus side, from Barr’s experienced point of view, when Obama was challenged on a similarly weak assertion of presidential privilege, it took the courts four years to decide that his weak-ass defense could not stand.
A random note about time: four years is the difference between Mr. Hitler as Time Magazine Man of the Year, for restoring German power and prestige, and Mr. Hitler as a Hall of Fame mass murderer at the top of his game, gleefully slaughtering in the East.
Back to contempt of a subpoena to testify, or to release documents.
Chelsea Manning, who leaked, among other things, video footage of American soldiers committing war crimes, murdering civilians and civilians who went to rescue the surviving machine gunned civilians, with the full backing of their commander (“light ’em up!”), was locked up for seven years, after being sentenced to thirty-five years for giving the files to Wikileaks. She recently was locked up for more than two months for failing to comply with a grand jury subpoena (she got out yesterday or today, temporarily). They are trying to force her to give additional testimony that will aid in convicting Mr. Wikileaks, Julian Assange, and locking him up under the jail for the rest of his natural life.
Meanwhile, this smug, openly contemptuous, expertly evasive fuck has already shown his eagerness to wipe his ass with any and all legal process.
He openly advises the president, in writing, on how to best stonewall to drag out the legal processes against him for as long as the law allows, including using a pathetically weak constitutional defense to support Trump’s order to everyone in government (and those formerly in his government) to ignore all subpoenas and not give anyone anything under any circumstance. Trump has vowed to fight all efforts at transparency all the way up to the Supreme Court, a court he is betting will vote on strict partisan lines to protect the corrupt bastard who appointed the two most recent right wing die hards.
As for contempt of Congress itself, as a matter of law and practice … 
Trump clearly has a very guilty conscience. You would too, if you’d done what he has done for his entire life. I n fairness to him, though, he has never been held accountable for any of the many incredibly stupid, immoral, vicious, inexcusable and illegal things he has done since childhood. When your father is a billionaire with a weakness for bailing you out of your own colossally imbecilic fuck-ups, year after year, decade after decade, you don’t really have to worry. You never have to worry!
Ask Don Jr, ask Ivanka, ask Jared. They will tell you, in no uncertain terms, “it depends”.
 You can read that basics about Contempt of Congress HERE.
There’s a good discussion of what Congress can do to give teeth to a contempt citation HERE
A person cited for contempt of Congress can be arrested by the sergeant-at-arms of the House of Representatives and dragged into Congress. The last time this procedure was used was a long time ago. Gratifying as it would be to see Barr dragged into the House in handcuffs, it would just as likely be a bloodbath as a cornered, twitchy, enraged Barr could use his own armed agents to repel the sergeant-at-arms in a hail of bullets. I would put nothing past the smug, contemptuous Nazi bastard.
Here are a couple of powerful government appointees who were held in contempt in recent times (from the table at the bottom of the Wiki):
Eric Holder, Attorney General under Obama, was found in contempt of Congress (by a vote of 255-67, no less) in 2012.
Note also that Anne Gorsuch, mother of our own Neil Gorsuch, was, like her son, a faithful soldier whose service to her masters included allowing herself to be cited for contempt of Congress.