I think you are wrong, because half the country is not consevative, did you forget?
The country doesn't divide in half by political philosophy. A strong grassroots movement with motivated voters campaigning can sway voters. But if you take a candidate like Giuliani who loses the Gun voters and loses the Abortion voters - both of whom are strong activists - the motivation of the grassroots campaign is weakened.
McCain is the same. Romney may or may not be able to move activist voters.
Conservative principles appeal to more people than not. When we campaign on true conservative principles, we win elections. Ask Ronald Reagan. Ask Newt Gingrich.