Barack Obama’s public approval rating has dropped to as low as 47% in the last week, according to Gallup. Although the President will not appear on the ballot again until 2012, how the public views his presidency will have a direct impact on each party’s performance in next year’s mid-term elections.

The party holding the White House has lost seats in 10 of the last 12 mid-terms, going back to President Kennedy’s 1962 losses. Even in that year, with a 74% approval rating following the resolution of the Cuban Missile Crisis, Kennedy’s Democrats lost seats in the House. Historically, the public uses mid-term elections to correct for the perceived excesses of the party in power, while the absence of coattail effects may result in some seats reverting back to the party with the natural advantage in the district.

IMPACT ON CONGRESSIONAL RACES. The magnitude of the net losses suffered by the President’s party in Congress has been in direct, inverse proportion to the President’s public approval rating on Election Day. The party in control of the White House suffered the most in 1966, 1974 and 1994 when the incumbent’s approval ratings were all under 50%. High approval ratings of President Clinton in 1998 (66%) and President Bush in 2002 (63%) helped the governing party gain seats in those two years — a historical aberration.

IMPACT ON STATE RACES. The spillover effect of Presidential approval directly impacts state legislative contests as well. The magnitude of losses suffered by the President’s party in legislative races has been similar to those in Congressional contests. The public has demonstrated it will hold state legislative candidates of the President’s party accountable for the President’s actions. The same three cycles when incumbent Presidents were at their lowest mid-term approvals also saw the greatest net loss of state legislative seats for the President’s party.

Lyndon Johnson’s Democrats lost 762 legislative seats in 1966 when Johnson’s approval stood at 49%. Republicans lost 628 seats with Richard Nixon at 47% in 1974. While Republicans took control of Congress in 1994 due in part to Bill Clinton’s 46% approval, Democrats also lost 514 state legislative seats that year.

AVERAGE LOSSES. When the President’s approval rating has been above 60% on Election Day, the average net loss has been zero House seats and 15 state legislative seats. At between 50% and 59%, the President’s party has on average lost 12 House seats and 217 legislative seats. And an approval rating under 50% has typically resulted in a wipeout of 41 House and 477 state legislative seats lost by the President’s party. Barack Obama’s approval rating within the last week has hovered between 47% and 50% in Gallup’s surveys.

Presidential approval ratings alone are unlikely to result in any lopsided district changing parties. However, the impact in marginal districts (such as those held by Democrats but won by John McCain in 2008) can potentially be sufficient to impact the final outcome.