Sen. Susan Collins (R-Maine) is emerging as the most important vote in the Senate battle over President Trump’s impeachment trial, as Democrats regard her support as key to getting the additional witnesses and documents they need to build their case. Collins, who has played a pivotal role in the biggest Senate debates of the Trump era, finds herself once again in the spotlight as one of the Republicans considered most likely to side with Democrats. “She’s an important player,” said Senate Democratic Whip Dick Durbin (D-Ill.), who noted that while every Senate vote is important, key trial questions will be...