He got it (incorrectly) from the constitution. The president is immune to almost all forms of prosecution - but not the kind that is called "Impeachment." You cannot arrest the president for a traffic violation, for example.
As to the evidence pointing to an attempt to influence the election, here is my question: IF Joe Biden really DID try to bribe off the prosecutor who was looking in potential misdeeds and malfeasance of his son Hunter Biden while he was managing a Ukraine government contract, then Trump was faced with a dilemma. He could not legally send USA law enforcement to the Ukraine to investigate that situation because there is no law in the USA that relates to US citizens attempting to bribe foreign officials. (There IS a law for US corporations - but not for individuals.)
So the question is this: Trump, because the Department of Homeland Security reports to him, is in effect the chief law enforcement officer of the USA. Even the USA Attorney General reports to him. He has a duty to protect the constitution. If he learned of public bribery by a person such as Joe Biden in a foreign country, and if that person might become the next president, does he or does he not have an obligation to root out crime of a very high level. Let's face it. If someone tried to bribe a U.S. federal attorney, wouldn't you want to know about that?
So anyway, that phone call was made to root out a purported wrong-doing by Joe Biden. And it doesn't matter if later that story was found to not be true. The question is, "What was known and what was rumored at the time of the phone call?"
Sen. Lindsey Graham said that former Vice President Joe Biden and his son Hunter have escaped scrutiny so far but should be "looked at" for the pair’s relationship with Ukraine.The South Carolina Republican told reporters in the Capitol on Thursday that a double standard has allowed the Bidens...
www.washingtonexaminer.com
Burisma’s connection to Hunter Biden has made it much harder for Ukrainian authorities to investigate the company for corruption
time.com
There are those who want to say that all of the charges have been debunked. And that might be true. But if the charges were still uncertain at the time then I think Trump would have been right to follow up on it.