The case on substance is weak. Why can’t the President ask a foreign leader to investigate corruption? To make their case, the Democrats will have to defend the indefensible payments made to Hunter Biden. McConnell could subpoena Hunter Biden and bring in a host of Ukrainian witnesses to testify about what why they were paying him.
Friendly testimony by the Ukrainian President and other Ukrainian officials asserting the call was proper will undercut the primary contention that there was something wrong with the call. When you read the actual transcript and not the Schiff parody, you do not find any threat to cut-off aid unless dirt is dug up and or that aid will stop unless opposition research is done against Joe Biden. It just isn’t there. You do find concerns expressed about corruption and about the paltry amount of aid provided by Germany and other European countries. Trump does ask a favor, to look into Crowdstrike and the hacking of DNC servers in the 2016 election. So it is Trump who wants an investigation of foreign interference in US elections. Only later does Trump also suggest the Ukrainian President look into corruption involving the Bidens. No threats were made. The only person who threatened to cut-off aid to the Ukraine was Joe Biden. He bragged about giving the Ukraine a six hour deadline to meet his demands to fire a Prosecutor investigating corruption at the company making payoffs to his son. It doesn’t take an ethics expert to see the Biden call as a blatant conflict of interest far more serious and improper than anything in the Trump call. In the war of calls, Trump comes out the better.
McConnell could also subpoena the Whistleblower and Adam Schiff. These could cast the whole Ukraine call episode as a partisan plot to overturn the election of 2016.
In the end, because the Democrats have pursued this in such a partisan fashion, the chances are very low of getting more than a few Republican Senators on their side. The Republican base will regard those defectors as traitors and punish them with political oblivion. On the other side, I see at least two and maybe five Democrat Senators who will not vote to convict. This will turn into an outright majority vote exonerating Trump, nowhere near the necessary two thirds vote to convict.