I went looking for this story and it only came up via Washington Examiner's reporting from the House Foreign Affairs Committee, which basically sums up their letter, and the Daily Beast, which has some red flags in it like:
"...Eliot Engel on Monday expressed his alarm over the Trump administration’s potential plans"
"Withdrawing from the treaty could upend this transatlantic alliance, and place Ukraine at greater risk."
"...expressing his alarm over Trump’s reported plans to leave the treaty."
Nah, there's a story there, I'm just not sure what source the Daily Beast was referring to and since they're attached at the hip with the Democrats I'm left wondering if this is a genuine concern or an attempt to layer on some weight to the Trump/Zelensky phone call that somehow turned into an impeachable offense.