It’ll probably be only a matter of time before the jokes start circulating about Senator Obama and Senator Kennedy. Mr. Kennedy has become a symbol in some circles on the right of personal foibles and out-of-touch Massachusetts left-wing extremism. But as is often the case, the reality is more complex than the stereotype, which is why Mr. Kennedy’s endorsement yesterday of Senator Obama has the potential to provide more than a one-day boost. At his best, Senator Kennedy has worked across party lines to get things done in Washington. There was the Kennedy-Kassebaum legislation that allowed employees the option of continuing their health insurance when they left their jobs. There were the economic sanctions on Iran and Libya, known as the Kennedy-D’Amato Amendment. There was the McCain-Kennedy legislation — yet unpassed — to bring millions of illegal immigrants out of the shadows and give them a path to citizenship. There was the compromise Senator Kennedy reached with President Bush on No Child Left Behind, a law that raised standards, increased testing requirements, and provided modestly increased competition in public education. If that is the legacy of Senator Kennedy that Mr. Obama embraces, his presidential campaign has the potential to make unity more than a slogan, and to make the Kennedy endorsement a story that matters for longer than a 24-hour news cycle.