UPDATE: Here's a list of key legislation where the GOP and Democrats differ.
Republicans MUST TURN OUT AND VOTE in key races and try to hold onto a slim majority in both Houses (or at least one, in which case I'd prefer the Senate for the sole reason of being able to push through -- possibly -- some more conservative jurists).
I recall very clearly in 1992 when a very dear conservative friend of mine evinced disdain for Bush the father essentially saying "Let's elect a Democrat and we'll get a real conservative in in four years." Anyone Republican who is thinking that this year is STUPID, STUPID, STUPID. 1992 brought us eight dissolute years of Clinton.
Here is the best reason why Republicans need to get out and vote, holding their noses if they must:
Key House Committees/current (GOP) chair/likely Dem chair
the numbers in brackets show the rating of each member by Americans for Democratic Action - a 'perfect' liberal gets 100.
Appropriations Lewis, CA  Obey, WI 
Armed Services Hunter, CA  Skelton, MO 
Energy Barton, TX  Dingell, MI 
Financial Serv Oxley, OH  Frank, MA 
Govt Reform Davis, VA  Waxman, CA 
Homeland Sec. King, NY  Thompson, MS 
Judiciary Sensenbrenner, WI  Conyers, MI 
Rules Dreier, CA  Slaughter, NY 
Ways n Means Thomas, CA  Rangel, NY 
Speaker Hastert, IL  Pelosi, CA 
Whip Blunt, MO  Hoyer, MD 
A Democratic takeover in the House would mean San Francisco's Pelosi as Speaker (most likely) and a veritable 1970s freak show of unrepentant liberals like Dingell, Waxman, Conyers and Rangel running some of the most powerful committees in the House. And you can bet that Conyers will try to impeach the President.
And, in the Senate:
Appropriations Cochran, MS  Byrd, WV 
Armed Services Warner, VA  Levin, MI 
Banking Shelby, AL  Sarbanes, MD* 
Budget Gregg, NH  Conrad, ND 
Energy Domenici, NM  Bingaman, NM 
Finance Grassley, IA  Baucus, MT 
For' Relations Lugar, IN  Biden, DE 
Health, Labor Enzi, WY  Kennedy, MA 
Judiciary Specter, PA  Leahy, VT 
Rules Lott, MS  Dodd, CT 
Intelligence Roberts, KS  Rockefeller, WV 
If you don't want to go to bed with Snarlin' Arlen, just imagine waking up with Dick Leahy! Democratic Senate leaders would include a rogue's gallery of Carl Levin, the narcissistic Joe Biden, Ted Kennedy and Christopher Dodd. Byrd will probably be Majority Leader. Paul Sarbanes is thankfully retiring but that probably means Dick Durbin will sneak in somewhere.
And the top items on the Democratic agenda:
1. Obstruct, obstruct, obstruct
2. Cut and run in Iraq - just blame the mess on Bush
3. Roll back tax cuts
4. NO conservative jurists
5. Become more internationalist -- Make nice to the UN and follow meekly
6. Impeach Bush