Susan Collins, the Republican Maine senator seen by Democrats as vulnerable in November’s midterm election, has revealed a decades-old medical condition she said affects her appearance, but not her ability to do her job. Collins, 73, told News Center Maine , an NBC affiliate, that she has a benign essential tremor she treats with medication, which sometimes causes her hands, arms and head to tremble. “I have had it for the entire time that I have served in the United States Senate,” Collins told the outlet, adding that it was an “extremely common” condition. “It has absolutely no impact on my ability to do my job or on how I feel each day,” she said. The health of Collins – first elected to the US Senate in 1996, and the longest-serving Republican woman in the chamber – has come under recent scrutiny. She is currently serving her fifth term of office in a state that Democrats are eyeing as one that could flip control of the Senate in November.…