Democrats haven't given up on impeaching President Donald Trump now that Special Counsel Robert Mueller's investigation found no evidence of impeachable offenses. They can't let go of what they need to be true-the president is a fraud and a crook.

Part of this apparent obsession is politics posturing. Another part, though-irrational bias - is more worrisome.

The impeachment process is a political maneuver, not a criminal court proceeding. It rarely leads to removal from office. While Democrats would dance in the streets if the Mueller investigation had found evidence of treasonous activity, a sitting president has never been charged with treason. The bar is extremely high for that charge to stick. And the hope that collusion would in some way fall into that category defies reason.

The accepted legal definition of "collusion" is a secret agreement between two or more persons, who seem to have conflicting interests, to abuse the law or the legal system, deceive a court or defraud a third party. The two-year, multimillion-dollar Department of Justice investigation found nothing close to that.

Even if collusion existed in the president's past, it would have had to occur during his presidency, not before it, to be criminal. Digging dirt on an opponent not yet elected would hardly be a first in politics. And two witnesses must testify under oath to prove intent to commit a crime. Mr. Mueller found no one to do so, pushing treason off the impeachment table. That leaves other high crimes and misdemeanors to consider.

The terms "high crimes" and "misdemeanors" were chosen by the Framers of the Constitution because of well understood behaviors under British law. In addition to bribery, the worst of those behaviors in the 1700s were misappropriating government funds, appointing unfit subordinates, not spending money allocated by Parliament, threatening a grand jury, disobeying an order from Parliament, arresting a man to keep him from running for Parliament, and granting warrants without cause.

Bribery has never been on the "get Trump" agenda. No one alleged that money changed hands for political favors, so obstruction of justice during Mueller's investigation is the opposition's next best bet. But Mr. Mueller couldn't find two witnesses to support that allegation. So, the House Judiciary Committee chair thinks Mueller's 60-person team must have missed something. How is this not faulty reasoning?

Impeachment is a condemnation for alleged criminal behavior. Even if a House majority votes to impeach a president, only the U.S. Senate can try that president to remove him or her from office. In 1998, the House, along party lines, impeached President Bill Clinton for lying under oath and obstructing justice. They had plenty of evidence to back it up. Still, the Senate acquitted him and he remained in office.

The Democrat push for another investigation of President Trump has one realistic objective - causing the president's voters to lose confidence in him, thereby increasing the likelihood of a Democrat victory in 2020. But why the bulldog approach?

Social science describes many human cognitive biases that warp our perception of reality, preventing us from thinking rationally. One bias is the "backfire effect." When our core beliefs are challenged, being wrong about them can seem like a personal attack on self or group identity. We double down on our beliefs, refusing to accept evidence to the contrary. Mark Twain described this in plain language. "It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."

To many of us, this is exactly what is driving the Democrat bus. Spending more taxpayer money to chase a chimera is crazy. But it also excuses the chasers from exercising their responsibilities in Congress. This is not crazy. It's unethical, plain and simple.