THE problem with Washington, according to Sen. Russell Feingold, is too much free crumb cake. Every day, explains the Wisconsin Democrat, he receives a handful of gifts from special interest groups - especially those with business pending in Congress. In addition to crumb cake, Senator Feingold has been wooed with books, CDs, peanut brittle, spark plugs, cuff links, bratwurst, denture cream, and a ceramic milk jug. Last year, the National Rifle Association sent the senator a cookbook. To Feingold, free gifts, meals, and trips perpetuate a "circle of influence" in Washington that skews policy debates and frustrates voters. Increasingly, his colleagues appear to agree: The …