It is better that we lose to the Democrats than a Republican win with RINOs. RINO stands for Republican In Name Only, it refers to liberals who are chosen by the republican party as their candidates, that is done because republicans have been operating under the quite flawed policy that stated "Pick the most conservative candidate that can win". So they will pick a highly liberal candidate who is slightly less liberal than the opponent to run in liberal states, for the purpose of securing a Republican victory.
This is a disastrous policy!
Neither history books nor the man on the street knows whether the conservatives or liberals are in power... what they know and record is which party is in power. With RINOs you can have the utterly disastrous situation of a Republican majority, but a conservative minority. Where a republican congress is passing things like bail outs, hand outs, pork, spending, tax hikes, nationalization, and other socialist and harmful legislation. People will look and see mounting debt and spending and will say "gee, conservatives don't know how to manage the economy". Even though it is not conservatives, it is a republican congress with a liberal majority.
We need to have blame properly assigned. If the republicans win the coming election with RINOs than they will NOT be able to fix anything, and will take blame for a continuously worsening economy, and then in 2012 democrats will come back into power. Sure, it will be disastrous for Democrats to remain in power for 2 more years... but it will be better in the long term. We could properly assign blame, people and history books will show that socialism, uncontrolled spending, and rising taxes harm the economy, and we could properly take control in 2012 and actually fix the problems we face.
While the above is the biggest issue and reason to lose rather then win with RINOs, it is not the only reason. The other is that the endorsement of RINOs is seen as a betrayal of conservative principles, it leads people to say that "the two parties are really mostly the same" and results in conservatives who simply refuse to vote at all. It reduces the trust of people in the republican party, as they are seen as sellouts who care nothing for the people's plight, only for their own personal authority.