Republican favorability has declined 29% of the last 16 years, while Democratic favorability has improved by 10%.
Republican decline in net favorability over the past 16 years:
1993 19%
1997 11%
2001 7%
2005 -7%
2009 -10%
Democratic Favorability over the same period:
1993 +24% Delta = +5
1997 +12% Delta = +1
2001 +24% Delta = +14
2005 +13% Delta = +20
2009 +5% Delta = +15
(Taken from Roper poll archive of polling done one year before midterm elections.)