Do members of Congress automatically have security clearances needed to do their jobs? There will be follow-ups depending on the answer.
Yes, in a way. They do not hold a specific security clearance, nor are they subject to a backgound check, but are given access to classified information by virtue of their constitutional office.
The House leadership has required that members take a non-disclosure oath, I believe at the beginning of each Congress. I am pretty sure that Senators do not have to take any such oath. There are a few of liberal Democrats who have made a big show of refusing to take the oath, and they have been refused accessed by the House leadership to classified briefings. See here for a list of those who have taken the oath.
And I just found this, which may answer most of your questions.