Despite forecasts of a blue tsunami, it’s still not guaranteed that Democrats will win back the House majority. But the playing field of competitive House races is expanding and shifting to almost exclusively Republican territory.

After the latest round of changes, Inside Elections now has 68 Republican seats rated as vulnerable compared to just 10 vulnerable Democratic seats. And there are at least a couple dozen more GOP-held seats that could develop into competitive races in the months ahead.

That discrepancy in the playing field is reminiscent of previous “wave” elections. In April 2010, there were 68 vulnerable Democratic House seats and 11 vulnerable Republican seats. Republicans gained 63 seats later than year. And in May 2006, there were 42 vulnerable Republican seats and 11 vulnerable Democratic seats. Six months later, Democrats gained 30 seats.

Of course, just because a seat is vulnerable, it doesn’t mean a race is lost. But the larger number of vulnerable seats means Democrats need to win a smaller proportion of the total competitive districts to gain the 23 seats necessary for the majority.