Libertarianism is offshot of liberalism.
Liberalism rn is very wide contain both right and left ideas, no matter what (except ultraright ideas): capitalism, democracy, conservative, liberty, etc. What means its a perfect idealogy for those who cant read anything tougher than Atlas Shrugged (dunno), dont know the theory of both wings and thinking that there is some way to seat on both chairs (democracity and capitalism).
Cant say american democratic party somehow linking with progressive in practice. Both conservative (republicans more than others but still).