That is a leftist conspiracy theory.
There were dozens of people listening in on that phone call. There was an impeachment and a senate investigation which determined Trump was not guilty.
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There was no conspiracy. The text of the call was made available for everybody to see. He was found not guilty of the charges in the impeachment articles, but the call made it clear he was asking Zelensky to investigate Biden. Whether it was "blackmail" or "extortion" in return for the money is not as important as his request of a foreign leader to investigate a potential political opponent. Completely inappropriate behavior for an American president.