In the last offering in these pages, your humble servant submitted for your incisive consideration that President Trump can be impeached, and convicted, for stealing from his charitable non-profit before he took office.  It is to be admitted, however, that people often think of impeachable offenses as misdeeds committed while in office, and actions committed in connection with the performance of that office.
We really need to consider that problem no further than to speculate about what could be done about a dead body found buried in a president’s former residence. Still, it will be useful to take a look at the pertinent constitutional provision to see if the impeachment process is so restricted. Article II, Section 4 of the Constitution reads this way:
“The President, Vice President and all civil Officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors.” 
There is no restriction on when the treason, bribery, or other crime is committed. And, really, we should be grateful for that. Otherwise, a failed attempt at treason, only discovered after the perpetrator took office, would be without an immediate remedy in the case of the President; who, according to prevailing wisdom, cannot be prosecuted until his term is completed.
But there is another impeachable offense that the president has committed in office, beyond the extortion attempt on the president of Ukraine that congressional Democrats appear to be hanging their hats on. He revealed state secrets to the Russians! 
“The intelligence disclosed by Mr. Trump in a meeting with Sergey V. Lavrov, the Russian foreign minister, and Sergey I. Kislyak, the Russian ambassador to the United States, was about an Islamic State plot….” At the time that happened in 2017, the prevailing wisdom was that Mr. Trump’s disclosure wasn’t illegal, since “the president has the power to declassify almost anything.”
To the extent that is true, it isn’t a complete analysis. As the Lawfare blog pointed out at the time,
“Questions of criminality aside, we turn to the far more significant issues: If the President gave this information away through carelessness or neglect, he has arguably breached his oath of office….in taking the oath President Trump swore to ‘faithfully execute the Office of President of the United States’ and to ‘preserve, protect and defend the Constitution of the United States’ to the best of his ability. It’s very hard to argue that carelessly giving away highly sensitive material to an adversary foreign power constitutes a faithful execution of the office of President.
“Violating the oath of office does not require violating a criminal statute. If the President decided to write the nuclear codes on a sticky note on his desk and then took a photo of it and tweeted it, he would not technically have violated any criminal law–just as he hasn’t here. He has the constitutional authority to dictate that the safeguarding of nuclear materials shall be done through sticky notes in plain sight and tweeted, even the authority to declassify the codes outright. Yet, we would all understand this degree of negligence to be a gross violation of his oath of office.” 
Still, we need not concede the illegality point so readily. Federal statute provides that whoever “Whoever knowingly and willfully communicates, furnishes, transmits, or otherwise makes available to an unauthorized person, or publishes, or uses in any manner prejudicial to the safety or interest of the United States or for the benefit of any foreign government to the detriment of the United States any classified information…obtained by the processes of communication intelligence from the communications of any foreign government, knowing the same to have been obtained by such processes” is subject to a fine, imprisonment up to ten years, or both. 
Now the information Mr. Trump disclosed had come from an ally, probably Israel, undeniably a foreign government.  Classified information for these purposes has the statutory definition of “information which…is, for reasons of national security, specifically designated by a United States Government Agency for limited or restricted dissemination or distribution….” That means that once the appropriate agency designates information declassified, it retains that status until the agency declassifies it. Of course, the President has the power to order the information declassified. But Mr. Trump didn’t do that in this case; he simply disclosed the information. To the Russians.
Thus, it appears that the President is capable of violating this law after all. And this he manifestly did. His disclosure to the Russians of classified information should be yet another article of impeachment to be brought before the Senate for trial.