Here are the Democratic senators up whose will be up for re-election in '08, if they choose, etc:
From the US Senate site,
http://www.senate.gov/general/contact_information/senators_cfm.cfm?OrderBy=party&Sort=ASCBaucus, Max- (D - MT) Class II
511 HART SENATE OFFICE BUILDING WASHINGTON DC 20510
(202) 224-2651
Web Form: baucus.senate.gov/contact/emailForm.cfm?subj=issue
Biden, Joseph R., Jr.- (D - DE) Class II
201 RUSSELL SENATE OFFICE BUILDING WASHINGTON DC 20510
(202) 224-5042
Web Form: biden.senate.gov/contact/emailjoe.cfm
Durbin, Richard- (D - IL) Class II
309 HART SENATE OFFICE BUILDING WASHINGTON DC 20510
(202) 224-2152
Web Form: durbin.senate.gov/contact.cfm
Harkin, Tom- (D - IA) Class II
731 HART SENATE OFFICE BUILDING WASHINGTON DC 20510
(202) 224-3254
Web Form: harkin.senate.gov/c/
Johnson, Tim- (D - SD) Class II
136 HART SENATE OFFICE BUILDING WASHINGTON DC 20510
(202) 224-5842
Web Form: johnson.senate.gov/contact/
Kerry, John F.- (D - MA) Class II
304 RUSSELL SENATE OFFICE BUILDING WASHINGTON DC 20510
(202) 224-2742
Web Form: kerry.senate.gov/v3/contact/email.html
Landrieu, Mary L.- (D - LA) Class II
724 HART SENATE OFFICE BUILDING WASHINGTON DC 20510
(202) 224-5824
Web Form: landrieu.senate.gov/contact/index.cfm
Lautenberg, Frank R.- (D - NJ) Class II
324 HART SENATE OFFICE BUILDING WASHINGTON DC 20510
(202) 224-3224
Web Form: lautenberg.senate.gov/contact/
Levin, Carl- (D - MI) Class II
269 RUSSELL SENATE OFFICE BUILDING WASHINGTON DC 20510
(202) 224-6221
Web Form: levin.senate.gov/contact/index.cfm
Pryor, Mark L.- (D - AR) Class II
255 DIRKSEN SENATE OFFICE BUILDING WASHINGTON DC 20510
(202) 224-2353
Web Form: pryor.senate.gov/contact/
Reed, Jack- (D - RI) Class II
728 HART SENATE OFFICE BUILDING WASHINGTON DC 20510
(202) 224-4642
Web Form: reed.senate.gov/contact/contact-share.cfm
Rockefeller, John D., IV- (D - WV) Class II
531 HART SENATE OFFICE BUILDING WASHINGTON DC 20510
(202) 224-6472
Web Form: rockefeller.senate.gov/services/email.cfm
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Edit: Site wouldn't sort by both class and party, so I have corrected.