Conservative and Republican are not the same thing!
There are liberal Republicans, opportunist Republicans who follow wherever the political wind blows, and conservative Republicans. There are also conservatives who are not Republican.
Otherwise, you would end up with folks like Chafee (before he left) and Snowe equated to folks like DeMint and Sanford.
You are correct. Republican and conservative are not the same thing and I do take care in separating the two.
However, I use them interchangeably in the one sense that the ONLY nationally viable political party representing us today is Republicans. Thus what happens to the Republican Party impacts us conservatives immeasurably.