It is acceptable for mainstream political parties openly to express xenophobic views. Freedom of speech allows for people to express whatever view they want, even if it is racist and prejudiced. Most political parties end up expressing view that are unacceptable to one group or another at some point in time.
I don't at all think that it's right that they can, but they sometimes do and not only "get away with it" but actually draw certain types of racists and xenophobic voters in with that kind of talk. It is mostly the conservatives who can get away with it. They most often say xenophobic things without an apology.
If anyone in a mainstream political party expresses xenophobic views, they will lose voters. Republicans in the United States have yet to embrace black and Hispanic voters, a move that will threaten any chance of a GOP member winning the White House in 2016. Xenophobia can be expressed by mainstream politicians, but voters will drop them and choose the other person or simply refuse to vote.
A mainstream political party, like people in leadership positions within politics, needs to stand for the people and should not discriminate. Xenophobic views just betray ignorance, so if they are expressed that should be a sign to vote the other way. But such speech is not "free" and can be legislated against.
Xenophobic means "having or showing an intense or irrational dislike or fear of people from other countries." (Google) I personally do not feel as though either of the political parties in the United States deal with this problem as a whole. I believe people can exhibit this behavior and there are probably people in both parties that may exhibit this problem, but overall I don't see it as a problem throughout either of our political parties. As far as them being able to openly express xenophobic views, I think they would find themselves alone quite quickly because very few rationally people would agree.