If the current election is a mid-term (as it will be this year) then the most previous election will have been a Presidential year and will skew the numbers. If you are running in 2008 and you go back to the previous election, you will be looking at a mid-term - 2006.
To see what is likely to happen in a Congressional election, you have to go back four years to measure potential turnout in the current cycle.
In 1998, the mid-term election in Bill Clinton's second term, the GOP was poised to pick up somewhere between eight and 12 seats. In the event, Democrats won a net of five seats and Speaker Newt Gingrich lost his job.
Democrats whipped up their supporters - especially African-American voters - into a frenzy of anti-Newt/anti-Republican activism and ended up with Presidential-level turnout numbers in enough Congressional districts to overwhelm what should have been a Republican tide.
Here's what the GOP should do: As soon as Karl Rove is cleared by the prosecutor investigating the Valerie Plame business - which may happen as soon as this week - he should leave the White House and take up a post at the Republican National Committee with two goals:
1. Raise money 2. Run the turnout operation for the mid-term elections.
On the first point, Karl Rove would generate tens of millions of dollars for Republican candidates across the country. Working out of the RNC he would not be subject to any of the rules which govern federal employees.
On the second point, Rove has raised turnout operations from a high art to a religion. Nobody, in our professional lifetime, has done it better.
Karl Rove on the stump for the next six months reminding the Republican faithful what their future will look like with Nancy Pelosi as Speaker of the House and John Conyers as Judiciary Chairman might well do for the GOP in 2006 what overplaying the impeachment hand did for the Democrats in 1998.
On a the Secret Decoder Ring page today: A citation from Time Magazine the week of the 1998 election which substantiates my point; a quandary-inducing Mullfoto, and a Catchy Caption of the Day.