Leaders from both major parties were in shock at the news this week that Massachusetts senator Ted Kennedy has a malignant brain tumor. A partisan lightning rod for many years, Sen. Kennedy has developed a remarkable reputation for working across party lines to address the nation's problems from his generally liberal perspective.

Election wonk that I am, I must admit that when I heard the news, one thing I realized was that Sen. Kennedy's health may lead him to step down from the Senate before his term ends in 2012. It turns out I wasn't the only one, as indicated by this Associated Press story on National Public Radio's site.

As background, the Massachusetts legislature in 2004 moved fast to pass a law requiring a special election to fill U.S. Senate vacancies rather than continue to have the governor appoint a replacement. Triggered by Sen. John Kerry's White House bid combined with the presence of Republican Mitt Romney in the Massachusetts gubernatorial mansion, this new law was partisan in its origins, but also was the right thing to do. It's a remarkable contrast that here in the 21st century most states accept the undemocratic process of gubernatorial appointment even as our constitutional framers in 1787 put provisions in the Constitution that have meant every single person who has served in the U.S. House has been popularly elected, including those filling vacancies. See my colleague David Moon's excellent commentary on this subject last year.

But there's one problem: the current vacancy law means involves primary elections followed by a general election. In the primary, the election will be held using plurality voting -- meaning the candidate who finishes first will take the nomination. With 10 Democratic U.S. House members potentially having a free crack at the seat, combined with other potential strong challengers like Sen. Kennedy's son Joe, you can imagine a wild free-for-all where the first-place finisher has well under 25% of the vote -- and potentially in a relatively low turnout race held apart from other major elections. Indeed, Massachusetts Democratic primaries for safely Democratic U.S. House seats have been won with as little as 23% in recent years. Tha can lead to a very unrepresentative result.

Once the primary winner takes office, that person could be there for a long, long time. It's in everyone's interest in Massachusetts to have its new U.S. Senator be grounded at least in majority support within his or her party. Massachusetts' legislature should pass a law requiring instant runoff voting for U.S. Senate vacancy elections - and other states should move to require Senate vacancies to be filled by popular election, ideally with instant runoff voting.

And of course we wish Sen. Kennedy well and hope that all this discussion remains a theoretical problem.