Dems didn't choose Cohen to testify. Trump chose him.
Think of a criminal trial. If the only eyewitness to the crime is the defendant's accomplice, a known liar with a grudge against the defendant, then whose fault is that? Not the prosecutor's; they usually discover the circumstances of the crime only after the fact. Rather, it is the defendant who chose this fellow to be an accomplice, and therefore an eyewitness.
Similarly, Democrats did not choose Michael Cohen to be involved in the events of which he is going to testify today. Trump chose him to be involved.
If Democrats don't have the luxury of choosing an honest, upstanding person to speak of Trump's illicit dealings, it is because Trump doesn't involve such people in his illicit dealings. (Or, it would seem, in any of his dealings!)
Remind your Republican friends of this as Cohen testifies today.
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Oh, and remind them that, while it's entirely proper to be skeptical of everything Cohen says today (I know I will), there's a difference between skepticism and dogmatic naysaying.
WITNESS: The sun will shine in D.C. tomorrow.
SKEPTIC: We'd better check the Weather Channel on that.
DOGMATIST: Don't check the Weather Channel, don't check anything, just grab your umbrella!
Encourage your Republican friends to be skeptics, not dogmatists.