Trump will deny that he did what Comey said he did. Trump's surrogates will also try to change the subject during Comey's testimony. They try to pin other things that Comey did, or did not do, in order to discredit his testimony. It then becomes Trump's word versus Comey's word. That will not work very well. Comey's testimony will be supported by his memoranda from his meetings with Trump. FBI agent memoranda from meetings with suspects is usually regarded as credible evidence by prosecutors and courts. If Coats also confirms that Trump asked him to influence Comey's investigation of Flynn, Trump's defense falls apart. It would be two trusted officials versus The Donald who has repeatedly demonstrated a strong indifference to the truth.
If the public witnesses the testimony, and reacts they way most normal people would react, it will put pressure on Republican leaders to abandon its failed president. They will have to decide what is better for their Party, and for their reelection prospects. Self interest works better than an appeal to the public interest for most Republicans. Trump will have made the same mistake that sunk Nixon. It would have been difficult to prove collusion between Trump's campaign and Russian agents. Trump's crude attempt to influence a FBI investigation will seem like obstruction of justice to the public. That is what sunk Nixon. His efforts to cover up his crime were exposed to the public and the leaders of his Party told him that they could no longer support him.