Republicans aren't perfect, but a government run by Republicans will translate to greater liberty versus the same government run by Democrats. The complaint from people to the right of the average Republican is that they are sick of voting for "the lesser of two evils" or for a "RINO", so (too many of them) either vote for a Libertarian or don't vote at all. The result, all too often, is that the Democrat wins. The only message this sends is that this district/state/country, given the choice between a liberal Democrat (there is no longer any other kind) and moderate Republican, they prefer the liberal Democrat. So, in the next election, the Republicans nominate someone even further to the left, and the cycle, or rather death spiral, continues.
But conservatives who vote this way are not voting in a principled way, they are voting as Democrats do - to make themselves feel good, or morally superior to the unwashed masses. The place for telling everyone about the wonders of libertarianism or conservatism is in Internet chatrooms or around the dinner table, not the voting booth. In the voting booth, in any meaningful election, there are two choices, and in every single case, the Republican is further to the right (will protect more freedom) than the Democrat. To the degree that more Republicans are elected in any given cycle, we will be more free, and to the degree that more Democrats are elected, we will be less free. Those are the only two choices, and those are always the only two choices. And then the next election cycle starts where the last election cycle left off. And so on.
The argument that the Republican and the Democrat are the same is often repeated, but it's always wrong. Even when the Republican is actually left of center (which is very rare, but Mike Castle in DE fit that bill), he or she is still to the right of the Democrat. Voting for the center or left-leaning Republican still translates to greater freedom than voting for the opposing Democrat. Even if the Republican and Democrat agree on 9 out of 10 issues, that still leaves no principled basis for a conservative to vote for any candidate other than the Republican; that one issue where they differ represents one freedom that the Republican will fight to uphold that the Democrat will not. It is still a choice between more freedom or less freedom.
The United States is never going to be as free as it once was, and it's certainly never going to be as free as it would be (briefly, until Russia and China conquered the planet while we watched and pontificated about how "it's not our war") if Libertarians ran it. The question facing principled conservatives each election is not whether we can return to the halcyon days of Grover Cleveland vetoing $40K in flood relief because the Constitution doesn't provide for it, but whether, for now, we choose to be a little more free or a little less free. Conservatives (too many of them, anyway) keep choosing "a little less free", and as a result we are a lot less free than we might have been. For awhile (the Reagan/Clinton generation), it worked in reverse - voters chose moderate Republicans over liberal Democrats and it was the Democrats who were forced to move a bit to the right. Now we have surrendered the advantage we had, and we'll continue slipping until we start playing the game correctly again. This coming election provides a perfect opportunity to start, and if conservatives don't show up to get Republicans in control of the Senate again, no matter which Republicans are running, it will probably be the last opportunity.