The House on Thursday approved creation of a joint House-Senate committee to coordinate responses to a terrorist attack on Congress.

The step came as lawmakers and political experts recommended a constitutional amendment to assure that Congress would continue to function after a disaster.

"We have to prepare for the unthinkable," said Rep. Martin Frost, D-Texas, co-sponsor of the proposal with Rep. David Dreier, chairman of the House Rules Committee (search).

The House resolution, which requires Senate approval, would establish a 20-member committee, equally divided by chamber and party. The committee would issue an interim report by next Jan. 31, and a final one four months later.

Concerns about the continuity of Congress grow out of the Sept. 11 attacks. It is widely believed that the terrorists who hijacked the plane that crashed in Pennsylvania may have had the Capitol as their destination.

The House has since made preparations for holding sessions outside the Capitol and has adopted rules on certifying the deaths of members. That allows the speaker to redefine what is a quorum and authorizes people other than the House leader to reconvene the House in the event the speaker is among those killed.

But Dreier, R-Calif., said there was no formal structure in place for the Senate and House to consider such issues as homeland security legislation and spending bills during times of crisis.

On Wednesday, a commission formed by two Washington think tanks, the conservative American Enterprise Institute (search) and the more liberal Brookings Institution (search), recommended that the Constitution be amended to assure the quick replacement of lawmakers killed in a terrorist attack.

Governors can fill vacancies in the Senate prior to an election, but the House requires special elections for vacancies. The concern is that it could take months to replace members of the House killed or incapacitated in an attack.

Rep. Brian Baird, D-Wash., has led efforts behind a constitutional amendment to allow governors to appoint representatives for a 90-day term if 25 percent of the House's 435 members have been killed or disabled or are missing and presumed dead. Replacements would not have to be of the same political party as their predecessors.

Also Thursday, Frost and House Homeland Security Committee Chairman Christopher Cox, R-Calif., outlined legislation to amend the 1947 Presidential Succession Act (search) to reflect the new age of terrorist threats.

Frost said that because of a "quirk" in that law, a Cabinet officer who becomes president if the president, vice president, House speaker and Senate president pro tempore are all killed could get bumped by a newly appointed Speaker.

Their bill would specify that only those holding the congressional leadership posts at the time the vacancy occurs in the offices of president or vice president could succeed to those offices.

It also specifies that the homeland security secretary would be eighth in line of succession to the presidency, behind the secretary of state, treasury secretary, defense secretary and defense secretary.

Traditionally, the line of succession parallels the order in which a department is created, but Cox said the homeland security chief should be placed higher because his job is "dealing with mortal threats to the country."