But doesn’t the Senate write the rules for an impeachment?
Why is McConnell saying he doesn’t have the majority votes to block calling new witnesses?
The rules of impeachment are in the Constitution. The House does the impeachment. The Senate judges whether or not what the House presents to them should result in the removal of a President. It is the job of the House to impeach by bringing forth an article of impeachment. They go through a discovery phase in order to do this. Whatever article of impeachment they put together and present to the Senate is then judged by the Senate. That article of impeachment is limited to the discovery produced to create it. If there is "new" discovery needed, the House is responsible for obtaining it, not the Senate. The Senate can make rules as to how their "judgement" portion of the process is handled but their rules cannot contradict the constitution.