WASHINGTON - Signaling a new phase in the struggle for control of Congress, House Democrats have reserved time for more than $30 million worth of campaign advertising this fall in roughly two dozen congressional districts, with a heavy emphasis on the Northeast and Midwest. The Democratic targets include clusters of Republican-held seats in the Philadelphia area held by Reps. Jim Gerlach, Curt Weldon and Michael Fitzpatrick, as well as the Ohio River Valley, where Reps. John Hostettler of Indiana, Geoff Davis of Kentucky and Steve Chabot of Ohio can expect a protracted televised barrage. Based on information available to date,...