The US House of Representatives has approved a "doomsday bill" allowing for special elections to be held speedily in case terrorists target Congress.
The 9/11 attacks forced a major strategic rethink
The elections would have to be held within 45 days, in the event that 100 or more members were killed.
It is suspected that the Congress building may have been the intended target of one of the four airliners hijacked on 11 September 2001.
The plane crashed in Pennsylvania after some passengers challenged the captors.
"Those passengers gave their lives to give us a second
chance," said Brian Baird, a Washington state Democrat.
The bill was supported by a majority of 306 representatives in the 435-seat body.
Some members argued it was inadequate, leaving the House of Representatives with too many empty seats for a long time in the event that an
attack causes mass fatalities.
They advocated a rival bill allowing temporary appointments, but this would require an amendment to the US Constitution.
Judiciary Committee Chairman James Sensenbrenner criticised the proposal, saying "democratic principles must be preserved at all costs".
Constitutional amendments in the US require a two-thirds majority in both chambers of the Congress and ratification by three-fourths of state legislatures.
The Congress has discussed, but never acted on, the continuity question during the Cold War in the 1950s and 1960s.