You can read it here.
The letter is by it's self not threatening, it was actions of said senators afterwards that some have a problem with. Others defend arm twisting by politicians.
https://www.redstate.com/elizabeth-vaughn/2019/09/26/read-shakedown-letter-three-dem-senators-sent-ukrainian-prosecutor-general/
I did not see the entire letter in the link.
I could only find the first paragraph of the letter on-line. It is fair to infer an implied threat by discussing less US support for the relationship while strongly encouraging cooperation with the Mueller investigation. But there is no smoking gun. The first paragraph of the letter was well drafted - its message was conveyed.
This demonstrates again how big boys act in the big boy world.
We are writing to express great concern about reports that your office has taken steps to impede cooperation with the investigation of United States Special counsel Robert Mueller.
As strong advocates for a robust and close relationship with Ukraine, we believe that our cooperation should extend to such legal matters, regardless of politics. Ours is a relationship built on a foundation of respect for the rule of law and accountable democratic institutions. In four short years, Ukraine has made significant progress in building these institutions despite ongoing military, economic and political pressure from Moscow. We have supported that capacity-building process and are disappointed that some in Kyiv appear to have cast aside these principles in order to avoid the ire of President Trump. If these reports are true,
we strongly encourage you to reverse course and halt any efforts to impede cooperation with this important investigation.
