The Campaign Finance Institute has issued this important report. It begins: “Independent soft money groups, — organizations that spend unlimited contributions in elections — are playing significant roles in 2008’s federal races. According to incomplete official and unofficial reports analyzed by CFI, federally oriented 527 and 501(c) groups have already collected or spent around $350 million and are on course to top $400 million. While 527 fundraising is down from its record 2004 total (but up from 2006), much of the gap has been closed by increased election activity on the part of 501 (c) groups. Soft money groups are mainly concentrating on a few dozen competitive races for the Senate and House, although some are also targeting the presidential election. The cumulative weight of such focused spending can be quite important in a tightly contested election.”
See also this post on the LA Times political blog.