After the War of Independence from Great Britain, the US had a very different form of government than the present one. This government functioned under the Articles of Confederation. This government had been formed in 1775 and had served the rebellious colonies fairly well throughout the war and into the initial years of peace and separation from the mother country across the sea.
Some people judged that government to be too loose an arrangement among the constituent states. A sufficient number of so minded people persuaded the states to convene a convention at Philadelphia to consider some amendments to the Articles of Confederation and to report these back as RECOMMENDATIONS to the state legislatures.
That did not happen. Instead the delegates to this convention seized control of the agenda and wrote a document that created a form of government in which there was an Executive Branch empowered in many ways to act without the direction given by the Legislative Branch. This Executive was made to be particularly independent in the conduct of war and and foreign relations. Some restrictions were established in that the military was to be funded by the legislature (if it chose to do so). The military was to be designed by the legislature and officers thereof were to be appointed by the senate on recommendation of the president. In foreign affairs the appointment of ambassadors and the approval of international treaties were made the responsibility of the senate as well, but both in war and in foreign relations the content and conduct of these government affairs were reserved to the Executive Branch. As an example of this, the Congress of the US had no role in running WW2.. The House of Representatives did not "sign off" on Operation Overlord or any other plan. The Congress did make an attempt to control military operations during the Civil War. A Joint Committee on the Conduct of the War was formed from among the most radical Republicans in both houses, but Lincoln largely ignored the machinations of this body.
Trump is to be tried for abuse of power and obstructing Congress. In the first instance he is accused of seeking political advantage by soliciting an investigation of the affairs of Joe Biden in a telephone call to the president of the Ukraine. His motivations in that call are unclear and are contested even among those who listened to the call in an official capacity. Biden was not then a candidate for office. He was a potential candidate. In the second article Trump is accused of Obstructing Congress. No president has ever been impeached on such a charge even though an inherent conflict between the Executive and Legislative Branches was built into the structure of the US Constitution in order to limit the power of both branches. For example; the president may wish to make some change in government practice that the Congress does not want. Many presidents have sought to obviate this difficulty by attaching signing statements to laws passed by Congress. These often say, in effect, "I am signing this but will not execute the will of Congress." No president has ever been impeached for doing that. Obama did that many times.
Speaker Pelosi has succeeded indicting Trump on such grounds and now seeks to control the trial pf the president in the senate through intimidation of members and such devices as accusing the Majority Leader of the Senate of being a Russian agent of influence "Moscow Mitch.". Her justification for that is McConnell's unwillingness to obey her.
Pelosi and company are now trying to remove a president on the grounds mentioned above. If they can do that, they will have succeeded in reverting the power structure within the federal government, reverting it to something much like the government of the Articles of Confederation. In that set up the federal government will become driven by the House of Representatives and will become the sole controlling part of the federal government with the ability to remove an opposition president through a simple majority vote and a rubber stamp trial in an intimidated senate. We will then have become a parliamentary democracy with the Speaker of the House controlling all.
Alan Dershowitz will testify in this wise at Trump's trial. I support his position. pl