It’s nearly Labor Day and there are slightly more than two months left in the presidential campaign, a high-stakes U.S. Senate race and an intense battle for control of Maine’s Legislature.

In other words, beware the political opinion polls. It is going to be hard to miss them.

“We’re getting more and more and more polls,” said Paul Lavrakas, a research psychologist and president of the American Association for Public Opinion Research. “I’ve been studying this since the mid-1980s, and every election cycle it seems like there’s more.”

As a pollster himself, Lavrakas doesn’t think more polls are necessarily a bad thing. Polls are a way to hear the opinions of actual voters during election seasons that are increasingly dominated by talking heads and partisan bloggers, polling experts say.

Even so, as the co-author of “The Voter’s Guide to Opinion Polls,” Lavrakas said it is increasingly important for voters to know the difference between a reliable poll and a poll that should be ignored.

Results of a poll on Maine’s U.S. Senate race made the rounds on political blogs last week and served as a reminder about the possible pitfalls of polls in election season.

The Charlie Summers for Senate campaign commissioned the poll, in part to see whether Summers had gained on independent front-runner Angus King after two weeks of anti-King TV advertising.

The poll suggested that King’s lead had slipped, from 28 points in a June poll commissioned by The Portland Press Herald to 18 points in early August. A higher percentage of voters said they were undecided, while Summers’ numbers increased slightly from one poll to the other.

The Summers campaign shared the poll results with bloggers in Maine and in Washington, D.C., who wrote that the race had tightened, citing a new poll but not saying that the Summers campaign had paid for it.

Then the Summers campaign sent out a news release and fundraising appeal, citing the media reports about new poll results. It also neglected to say the campaign had paid for the poll, although it didn’t hide the fact when asked.

The episode was a typical example of how poll results are manipulated by political campaigns, Lavrakas said. While the poll may be perfectly valid, a voter equipped with basic knowledge of polls would have known not to take the numbers at face value.

Here is what the experts say voters should know about three major types of polls: independent polls, internal polls and push polls.

Independent polls

Independent polls are the ones conducted by professional polling companies and paid for by groups that are not working for a candidate or a campaign. These are often commissioned by media organizations, such as The Portland Press Herald or ABC News or The Wall Street Journal.

Pollsters consider these to be generally the most reliable polls; but they aren’t perfect, and some are better than others.

“When it’s done well, it’s very scientific,” Lavrakas said; but he added, “when you are studying human behavior, you are never precise the way physical science can be.”

Polling science is always evolving. For example, the fact that more voters are switching from landline telephones to cellphones has posed new challenges for pollsters who want to get a truly random sample.

Pollsters now typically use both kinds of phones to reach voters.

Some aspects of polling, such as screening for likely voters or asking questions in a way that gets to a voter’s true feelings, can be more art than science, experts said. Even the order of questions can skew the results.

One universal mark of a good independent poll is full disclosure about when and how it was conducted. Also, polls always should disclose who paid for and conducted the surveys.

Internal polls

Internal polls are the ones paid for by candidates, campaigns and advocacy groups.

These polls often are aimed at gathering information to guide campaign strategies, and the results are usually not made public. Pollsters say internal polls can be just as scientifically reliable as independent polls. After all, they say, the people paying for them want accurate numbers, too.

However, polling experts caution about publicizing the results of internal polls. That’s because campaigns share only the results they like, while keeping most of their poll results secret, and the selection process creates a bias in the data.

Internal polls that are made public tend to give their candidates a roughly 6 percentage point advantage over independent polls conducted at the same time, researchers have found.

“In many cases, internal campaign polls are really good,” Shaw said, but added that “the ones that aren’t released are the most accurate. … I would interpret a leaked internal poll as a piece of advocacy.”

Push polling

Push polling, in truth, is not polling at all.

Push polls are a way to spread information to help or hurt a candidate or a campaign. They are not intended to gather information at all.

“The difference between a scientifically valid opinion survey and a push poll is that one is attempting to elicit opinions and one is attempting to change opinions,” Shaw said.

A group conducting a push poll may call thousands of voters rather than the hundreds needed to make a random sample, and it never actually tabulates results.

Push polling, in fact, is regulated in Maine as political advertising.