The same has been said of conservatives, ever since the 1964 Goldwater debacle. Reagan demonstrated the falsity of that proposition in his election landslides of '80 and '84.
That is not correct, as Reagan did not run for president as a libertarian, but as a conservative.
Reagan was never a member of the Libertarian party. Ron Paul, as a former Libertarian party presidential nominee is not electable as a national candidate.
A distinction should be made between Libertarian and libertarian. One is affiliated with a party, the other with an ideal. Even within the ranks of the party there is wide diversity of libertarianism ideals. Some are fiscally conservative and socially “liberal”. Some are both conservative. Hardcore libertarians are isolationists and even lean toward anarchy, survival of the fittest (Social Darwinism). Pure capitalism, i.e., lasier faire, is a libertarian type idea. Reagan had many such ideas, but some not-so-liberatarian ideas. Rarely do we see libertarians that are pure, all-the-way libertarians. Reagan was definitely a Republican, but many Republicans embrace libertarian ideas.