According to this Washington Post article, the Republicans are having trouble finding good candidates for the 2006 elections:

"With an unpopular war in Iraq, ethical controversies shadowing top Republicans in the House and Senate, and President Bush suffering the lowest approval ratings of his presidency, the waters look less inviting to politicians deciding whether to plunge into an election bid. Additionally, some Capitol Hill operatives complain that preoccupied senior White House officials have been less engaged in candidate recruitment than they were for the 2002 and 2004 elections."