Are political polls biased and why?

Great question! I think it's tough to avoid bias in the type, number, and phrasing of the questions (if you even try). 2nd, it's very difficult to target a truly random representative sample, because I think busier people who value their time more, so will be less likely to complete the poll, when contacted.

The problem by and large with political polls is the way the information is presented. Take the current presidential polls. Statistically, if I take a large enough random sample and present the choices in nonbiased manner (non-leading question phrasing, intonating my preference for the poll taker, counter balancing the answer choices, etc.) I should be able to accurately reflect the mood of the general population with the data that I have collected. However, it should come as no surprise that the major political polls are showing Trump and Clinton both well into the 40%'s. If I present you the question "are you more likely to vote for Donald Trump or Hillary Clinton" then you are forced into an A or B answer psychologically. If I ask you "in this election who do you plan on voting for?" Then you may say Gary Johnson, Jill Stein, or Mickey Mouse.

Not to sound like a conspiracy theorist (because it isn't a conspiracy that the "two parties" do not want to lose power) but if Gary Johnson is polling above 20% in some polls conducted at state levels and Stein polling at or near 8% in polls conducted at state levels...why do the polls that determine the debate stage have Johnson polling >9% and Stein polling at >5%?

A poll done properly can provide some insight. However, polls can very easily be manipulated and I believe the polls usually show the result that the people paying for them want it to show.

Yes. Polls have become a propaganda tool. They create an illusion of popularity for an unpopular candidate (In propaganda it's called the "Bandwagon Effect")

The results of a poll can be affected by so many factors that, unless one knows the details of the methods used in the poll, their results can be deceptive at best. Polls using exactly the same questions can vary greatly simply by varying the data gathering method. Door to door polls are influenced by location. Telephone polls are influenced by area code, source of the phone list, or type of phone being called( land line, cell, or both). Population polled (Democrats, registered voters, card carrying Republicans, etc) affect the results. Day of the week, phase of the moon or the number of flies in the ointment may also be factors.
And that's why polls need to be taken with a bag of salt(that's lots of grains).

If I ask the right series of questions in the right way and shoe horn the analysis, I can make it sound like most Americans believe in purple unicorns with pink polka dots, until I ask "do you believe in purple unicorns with pink polka dots?"

I'm nor for them really COL Lee Flemming, they always seem biased in one form or another. The only real poll I might remotely trust is the poll that declares a winner in November.

Plenty are by the Way they are worded to ensure a certain result. Others are done by where the Polling is done and in what Forum. I know that I have had fun with some of my Liberal Friends when we hear of one on a Conservative Forum that is an On-Line Poll we will gang up and Flip It from their Intended Result. Done that several times on FOX Polls and Town Hall Polls.

Most polls are biased unless they are truly taken from a neutral party. For instance, if I want to conduct a study to see if pot should be legalized, I'd probably conduct my poll around high schools and community colleges. If I were doing a study on female oppression, I might conduct my poll on the liberal arts campuses specifically around the women's studies classes.

There are many ways to sway a poll. Hell, if you went to a Star Wars movie premiere and asked the crowd which movie was better Star Wars or Star Trek, you can probably already guess the results.

People that do polling can get pretty accurate results. BUT there are pools to skew to one position or the other to influence . Push pools come to mind. As the date for election gets closer polls get more honest. The folks doing polls need credence . Nobody wants a polling company that missed the election by 20 points.