Because their terms last only two years, House Members in competitive districts must run permanent campaigns while carefully weighing every House vote for potential political pitfalls. Life in the Senate is usually more relaxed. However, as a consequence of recent turnover, some senators now face the prospect of having to defend their seats in 2010 and then gear up to defend them again two years later.

Gov. Deval Patrick announced this week a January 19 special election to finish the term of the late Sen. Ted Kennedy. It's a race that will likely be hotly contested, particularly on the Democratic side. And whoever wins the snap vote will not be able to rest for long since Mr. Kennedy's term expires in 2012. The turnaround will be even quicker for New York's Kirsten Gillibrand, appointed to replace Secretary of State Hillary Clinton in the U.S. Senate. Like most states where a governor appoints a replacement, she doesn't face a special election but must run in the next regularly scheduled federal election, i.e. November 2010. But since Clinton's term expires in 2012, Gillibrand must raise money and prepare for tough fights each of the next two cycles.

Given the strong Democratic profile of both states, Gillibrand and whoever wins the Massachusetts special election may have a harder time fighting off primary challengers than a general election opponent. That's why Democrats from President Obama on down have worked to discourage intra-party challengers to Gillibrand and perhaps will do the same for the Massachusetts winner. In one respect, Mike Bennett, the newly appointed senator from Colorado, has an easier road. He may be neither a primary nor general election shoo-in, but he only has to run once. Ken Salazar, whom he replaced when Mr. Salazar joined the Obama administration as interior secretary, was due to face voters next year anyway.