The Cook Political Report lists 27 Republicans as vulnerable. Their Democratic opponents have raised 60 percent as much money as the incumbents, the Washington Post reports.
In 2004, the report listed nine Republicans as vulnerable and their challengers had raised 42 percent of their collective war chest.
The Post said the Democrats also appear to be riding a trend, with contributions increasing even more when their candidates start to look like winners. For example, former U.S. Navy Vice Adm. Joe Sestak, running in Pennsylvania's 7th District in the Philadelphia suburbs, has raised more money in the last quarter than the man he hopes to beat, 10-term Republican Curt Weldon.
In what could be the most expensive race, Rep. Clay Shaw Jr., R-Fla., has raised $3.2 million, while his challenger, state Sen. Ron Klein, has brought in $2.7 million. Klein could bring in another $500,000 with a planned visit from former President Bill Clinton.