Andrew Gelman tries to look ahead to the 2010 midterms by first looking back at the Democrats’ share of the House vote over time:
From this picture, it looks possible but unlikely that there will be a 6% swing toward the Republicans (which is what it would take for them to bring their average district vote from 44% to 50%). Historically speaking, a 6% swing is a lot. The biggest shifts in the past few decades appear to be 1946–48, 1956–58, and 1972–74 (in favor of the Democrats) and 1964–66 and 1992–194 (for the Republicans). I don’t know if any of these would quite be enough to swing the House majority. A more likely outcome, if the Republicans indeed improve in next year’s election, is for them to make some gains but still be in the minority.
It is worth noting, as you can see from the chart, that in recent years there have been several occasions in the late-1990s in which Democrats won a narrow majority of House votes but the GOP controlled the majority of House seats. The drawing of district lines thus seems to favor Republicans. Since the lines will be redone after 2010, the electoral landscape may come to be more favorable for the Democrats once the census/redistricting process is done.