In a paper I have been working on for a long time, I have a character who, while testifying before Congress in the mid-1970s, had a stroke mid-sentence and died. This was apparently noteworthy enough that he got front-page coverage in the Washington Post. It got me wondering: in the 225 years or so that sessions of the US Congress have been convened, how many other cases are there of people who died while actually doing something in a session of Congress? I'd be especially interested in anyone who died mid-sentence, or any witnesses who died — just to see if this has ever happened other than my man in question.
The Congressional Cemetery was originally established by Congress to accommodate members who died in session at a time when it was not realistic to ship the body home for burial. Many others ended up buried there, but the list of notables - particularly the many members of Congress who landed there for the long sleep - includes all sorts of circumstance. I would begin there to research the question you are asking. This was recently listed as a National Historic Landmark District and the nomination, being a recent one, includes a great deal of useful information. I was involved in the listing of the resource, but I don't recall anyone who died midsentence, which is a great - and dramatic - way to go.
John Quincy Adams was a Congressman after he was president. The Congress was debating the Mexican American War, specifically a resolution honoring the victorious generals. He rose to seek recognition to speak, but immediately suffered a stroke and collapsed into the arms of a colleague. He died two days later on a sofa in the Speaker's office.