At the heart of the battles are -
For the Presidential Election - the states of Florida; Pennsylvania; Ohio; Michigan; Virginia; Missouri & Wisconsin.
For the Senate - the states of Alaska; Colorado; Maine; Minnesota; Mississippi; North Carolina; New Hampshire; New Mexico; Oregon & Virginia.
Key Districts which could be captured by the Democrats - Alaska at large; Alabama 02; Arizona 01; Colorado 04; Connecticut 04; Florida 08; 21; 24; Illinois 10 & 11; Louisiana 04; Michigan 07 & 09; Minnesota 03; Missouri 06; North Carolina 08; New Jersey 03 & 07; New Mexico 01 & 02; Nevada 03; New York 13; 25; 26 & 29; Ohio 01; 15; & 16; Pennsylvania 03; Virginia 02 & 11; Washington 08 and West Virginia 02.
There is a very useful site at http://usa.usembassy.de/government-elections.htm