Polls are the best way to find out who plans to vote and for whom they plan to vote. But polls are imperfect. They ask questions of a sampling of people — often about a thousand — and use those answers to draw conclusions about the public at large.

This year there is a new wrinkle, one that complicates the picture and could throw some of the polling off: the effects of newly enacted restrictive voting laws.

Take, for instance, the results of a New York Times/CBS News/Quinnipiac poll released Wednesday. “Likely voters” were polled in the swing states of Florida, Ohio and Pennsylvania, and President Obama led Mitt Romney in each state — by 6 points in Ohio and Florida and by 11 points in Pennsylvania. President Obama carried all three states in the last election and needs them in this one. Encouraging for him, right?

But let’s dig in a bit and look at some of the variables that could weigh on those results.

First, there are the quirks that always exist. It’s August and many voters aren’t intensely focused on the election yet. Sixty percent or less in each state say that they are paying a lot of attention to the presidential campaign at this point, and these are states that have been soaked in ads and visited often by the candidates. (On Wednesday, Obama made his 25th trip to Ohio since becoming president.)

People also tend to overstate their intention to vote. Many national and state polls show that more than three quarters of respondents say they will definitely vote in upcoming presidential elections. This is a major component of the way pollsters determine “likely voters.” But that level of voting is not supported by historical patterns. According to the United States Elections Project, the turnout rate for the voting-eligible population in Florida in 2008 was just 67 percent, in Ohio it was 68 percent and in Pennsylvania it was 64 percent. So many of those who say that they are definitely going to vote actually won’t.

Then there are the new voter restrictions that are likely to trim the voter rolls and add tremendous voter confusion.

Pennsylvania has passed a highly restrictive photo ID requirement for its voters. A study conducted by professors from the University of Washington and the University of New Mexico found that more than a million registered voters in Pennsylvania and 757,325 people who voted in 2008 lack a valid ID under this scheme. More than a third of registered voters are unaware that a photo ID law even exists.

This means that a lot of people who say that they are likely to vote may not actually be eligible to vote. (Arguments in a suit contesting the Pennsylvania law are being heard this week .)

Now to Florida and Ohio: both states have cut their early voting periods. According to the Brennan Center for Justice, more than a million people who voted in Florida and Ohio in 2008 did so on days that have been eliminated.

As the Associated Press reported about the Ohio restriction in July:

The state doesn’t track its early voters by party, so the stats don’t show exactly how much Obama might have benefited from early voting in Ohio. But both parties are sure he did. An extended voting period is perceived as benefiting Democrats because it increases voting opportunities for those harder to reach for an Election Day turnout — Hispanics, blacks, new citizens and poor people.

Florida has already moved to potentially purge thousands of voters from its registration rolls. In May, The Miami Herald said of the purge:

Hispanic, Democratic and independent-minded voters are the most likely to be targeted in a state hunt to remove thousands of noncitizens from Florida’s voting rolls, a Miami Herald computer analysis of elections records has found. Whites and Republicans are disproportionately the least-likely to face the threat of removal, the analysis of a list of more than 2,600 potential noncitizens shows.

The Republican governor of Florida has also made it harder for ex-felons to vote. According to a report last month in USA Today:

The Florida Board of Executive Clemency, headed by Republican Gov. Rick Scott, reversed predecessor Republican Gov. Charlie Crist‘s policy that automatically restored voting rights to non-violent offenders upon the completion of their sentences. Ex-felons must now wait five years before applying to regain rights.

The newspaper pointed out that “the Sentencing Project, a group advocating reforms in prison and sentencing policy, says 60% of the prison system population is made up of African Americans and Latinos.” It almost goes without saying that these groups traditionally vote more Democratic.

Rolling Stone reported in May that this could disenfranchise “100,000 previously eligible ex-felons” in Florida.

It’s unclear how many voters are aware of the new rules, and whether they’d be able to vote even if they were. What is clear is that fewer Democrats say that they are paying a lot of attention to the election in these three states than Republicans, by a margin of 8 to 14 percentage points. It would stand to reason that they might also be less aware of the new laws.

This year, we may have to take the polls with an even larger grain of salt than usual. The greatest margin of uncertainty may well be caused by poll respondents who think that they will able to vote for President Obama in November, but may not be allowed to do so.

And it’s all thanks to the Great Suppression of 2012.