In the post-Sept. 11, 2001, reality, we have seen military guards with M-16s patrol the Capitol and anti-aircraft artillery stationed around national monuments. It is no mystery that terrorists actively seek to interrupt our constitutional democracy.

The line of presidential succession, which determines who becomes president if both the president and vice president have died or are otherwise unable to carry out their duties, should be as solid as the concrete barriers lining the Capitol grounds. It is not. However, with a change in statute — not a constitutional amendment — Congress can ensure certainty in the line of successors, as well as continuity of federal policies.

Article II, Section 1 of the Constitution allows Congress to determine the line of succession to the presidency following the vice president. Congress last visited this issue seriously when it passed the Presidential Succession Act of 1947. Unfortunately, the 1947 act is ambiguous and we cannot afford ambiguity as to the identity and legitimacy of the president, particularly at a time of crisis.

The 1947 act is further flawed because it allows the presidency to be shifted to an opposing political party. This means if the vice presidency is vacant, our stock markets and foreign enemies will wonder whether an unfortunate event will result in a radical shift in policies; a terrorist might see an “opportunity” to radically shift our policies; and a partially or temporarily impaired president would think twice about taking a leave of absence under the 25th Amendment, or resigning, if either action would put the other party in control of all executive departments. Finally, third in the current line of succession is the President Pro Tem, a ceremonial position normally held by the longest-serving member of the Senate majority.

Current law provides that if the office of the vice president is vacant, the next in line is the Speaker, followed by the President Pro Tem. The recent “West Wing” season finale demonstrated how a president, under extreme duress could, at a time when there was no vice president, invoke the 25th Amendment and temporarily transfer control of the White House to a Speaker of the opposite political party. In real life, it is more likely that a president arguably suffering from temporary impairment would hang on to the presidency with the same tenacity that former Sen. Strom Thurmond (R-S.C.) held on to his seat at a time when his resignation would have handed his seat to an appointee of a Democratic governor.

Speaking of Thurmond, we should remember that just a few years ago, while in his late 90s, he was third in line for the presidency. Does this make sense in an era of suicide-assassins?

Here is a hypothetical designed to illustrate all the ambiguities of the 1947 act. The office of vice president, Speaker and President Pro Tem are all vacant. The president has nominated Ms. Smith to be the new vice president, and she awaits her confirmation hearings under the 25th Amendment. The House and the Senate have adjourned for the year, though Mr. Jones is serving as “temporary House Speaker” pursuant to House rule 1, clause 8 (3)(A). Now, imagine that the president dies. [IMGCAP(1)]

Does Mr. Jones, the temporary Speaker, become president? Probably not, but we’re not sure. In all probability, the secretary of State becomes acting president. But assume the House then reconvenes and elects a Speaker. Does that new Speaker then push aside the secretary of State and become the new president? What if the Senate elects a new President Pro Tem before the House elects a new Speaker? And what if Ms. Smith makes it through her vice presidential confirmation hearings — does she push aside whoever is then serving as president? Under this scenario, and under the ambiguity of the 1947 act, all five of the following could claim the presidency: Ms. Smith, Mr. Jones, the President Pro Tem, the newly elected Speaker and the secretary of State. Other, less contrived scenarios could create three or four claimants to the presidency. Even two plausible claimants to the White House is one too many.

Some will say that presidential succession has never gotten past a vice president, in part because Gerald Ford was confirmed promptly, before Richard Nixon resigned. But Sept. 11 shows that what is unlikely to occur naturally may well occur. In April 1865, John Wilkes Booth headed a partially successful conspiracy to assassinate President Abraham Lincoln and those who stood first, second and third in line to succeed him. Are we sure that al Qaeda can do no worse?

Next month, I will introduce the Presidential Succession Act of 2003, which is similar to legislation I introduced in March 2001. Under this legislation, the president will file an official document with the Clerk of the House designating, after the vice president, the next person in line of succession as either the Speaker or the House Minority Leader. Similarly, the president would file instructions with the Secretary of the Senate, designating the third in line as either the Senate Majority Leader or Minority Leader. (These designations can be revised if the majority becomes the minority.) The bill will further ensure certainty in presidential succession by clearly providing that if someone succeeds to the presidency, that person shall continue to serve until the end of the presidential term.

Our friends and enemies around the world, as well as the investment community, should know that similar policies will continue throughout a four-year term, and that the presidency will not be shifted to the other party by a tragic event. More importantly, the law should be absolutely clear so that whoever serves as president, particularly at a time of crisis, has unquestioned legitimacy. By acting now we can accomplish these ends. Or, we can just put this off until a problem arises.

Rep. Brad Sherman is a Democrat from California.