By Ken KlukowskiAmerican Civil Rights Union

President Obama is keeping the door open on prosecuting a federal appeals judge.A Democratic U.S. senator has called for even investigating the judge's possible impeachment. His crime? Signing a legal memo on fighting terrorism after the 9/11 attacks. This attack on the judiciary threatens the constitutional separation of powers, and must be emphatically denounced and opposed.

On Apr. 19, Sen. Claire McCaskill (D-Mo.), told Chris Wallace on "FOX News Sunday" that impeachment should be on table for Judge Jay S. Bybee, who holds a lifetime, Senate-confirmed appointment on the U.S. Court of Appeals for the Ninth Circuit. Sen. McCaskill was condemning Judge Bybee for legal memos that he signed when serving in the U.S. Department of Justice (DOJ), examining what powers the government had to combat terrorism.

Before becoming a federal appeals judge, Jay Bybee was the assistant attorney general in charge of DOJ's elite Office of Legal Counsel (OLC). The head of OLC is the top legal advisor in the federal government. The purpose of OLC is to examine legal questions put to them by the president or attorney general. OLC thoroughly examines the issue and then writes legal memoranda giving authoritative legal advice to the government on what the law requires.

As the assistant attorney general in charge of OLC, Mr. Bybee signed such memos, which are often classified if they deal with sensitive or national security matters. These memos necessarily involve difficult calls and controversial matters.

That is why only the finest lawyers ever serve in OLC, and especially in its leadership. Former Chief Justice William Rehnquist and Justice Antonin Scalia both led OLC before their appointments to the Supreme Court, and Justice Samuel Alito served as deputy at OLC. Other OLC chiefs include former Solicitor General Theodore Olson, widely regarded as the greatest Supreme Court advocate of this generation. These men are among the best legal minds this nation has ever seen.

Judge Bybee followed in this tradition, tackling the toughest legal problems this country faces in order to uphold the Constitution and rule of law. A former law clerk for the Fourth Circuit, Judge Bybee had a distinguished career in various divisions of DOJ and as a law professor before leading OLC and then being appointed to the Ninth Circuit. He has been confirmed by the U.S. Senate twice, both as the chief of OLC and also as a federal appeals judge.

This country has understood and upheld from its earliest days that we do not impeach federal judges because we disagree with their legal opinions. The Constitution grants judges lifetime appointments, and federal judges have only been removed for egregious criminal behavior, such as now-Rep. Alcee Hastings (D-Fla.), who as a federal judge was impeached and removed for corruption and perjury. In all of American history, only seven federal judges have been removed from the bench.

But the legal opinions in question here were not even issued by Judge Bybee as a judge. They were his work product as a DOJ lawyer. We do not make it a crime for lawyers to provide expert legal advice under even ordinary circumstances. It's outrageous to suggest that we should do so when it is a Senate-confirmed government attorney, responding to requests from the president of the United States about how he can respond to mass murder on U.S. soil.

Perhaps surprised by Sen. McCaskill's rhetoric, Chris Wallace directly asked her if she supported impeaching Judge Bybee. "I think we have to look at it," McCaskill responded.

This dangerously undermines national security and the constitutional separation of powers. OLC was tasked with developing a thorough legal analysis of how tough federal agents could be in questioning terrorists after the 9/11 attacks that killed 3,000 Americans. These confidential memos explored that difficult issue, and were just recently made public by the Obama White House in its efforts to radically change the policies that have kept his nation safe for almost eight years.

Judicial independence is critical to the separation of powers, an essential aspect of America's constitutional government for over 200 years. Federal judges are only removed for serious crimes. Judge Bybee's legal memos represent opinions held by many legal conservatives, and to criminalize them is to criminalize conservative legal views. What other conservative ideas will now result in federal judges coming under attack?

These threats of prosecution and impeachment, both of Judge Bybee and his former colleagues, are utterly appalling. It's unfortunately possible that Judge Bybee could be impeached by Nancy Pelosi's House, in a strictly partisan witch-hunt. But should that happen, there's no chance that even a simple majority of the U.S. Senate would convict and remove him, far short of the two-thirds vote needed.

If Democrats pursue this, they will pay a heavy price. If President Obama and Congressional Democrats want a fight over this, then a fight is what they will get.

Ken Klukowski is a fellow and senior legal analyst with the American Civil Rights Union, and holds a law degree from George Mason University.