WASHINGTON — The circumstances are eerily similar. In the middle of contentious health care deliberations, a larger-than-life figure in the Senate learns he has a very serious form of brain cancer. A leading voice goes quiet and the Senate suffers for it.

Such was the case in 2008 when Edward M. Kennedy, Democrat of Massachusetts, learned he had brain cancer and again this week when it was announced that John McCain, Republican of Arizona, had the exact condition that claimed his colleague — a man who shared Mr. McCain’s zest for a good argument and for cutting a deal.

“There are some very strong parallels,” said Senator Jack Reed of Rhode Island, who serves as the top Democrat on the Armed Services Committee, which Mr. McCain is the chairman of.

As colleagues of Mr. McCain from both parties acknowledged the vital role he has played in an institution where he is a dominant personality, they also said that his remarkable life story, most notably enduring torture for more than five years as a prisoner of war in Vietnam, made them believe he would return.