Editor's note: John P. Avlon is a CNN contributor and senior political columnist for The Daily Beast. He is the author of "Wingnuts: How the Lunatic Fringe Is Hijacking America."

(CNN) -- As the mosque debate dominates the summer spin cycle, a new Pew Research Poll shows that an increasing number of Americans believe that Barack Obama is a Muslim -- now up to 18 percent from 11 percent in March 2009. At the same time, only a third of Americans say Obama is a Christian, down from nearly 50 percent last year.

And while the biggest jumps in believing Obama is Muslim come from self-described conservative Republicans -- from 18 percent to 34 percent -- the number of Democrats who describe Obama as Christian fell from 55 percent to 46 percent. Independents -- while being the group least likely to want professions of religious beliefs from politicians -- have also fallen in with the drift.

I believe the American people are smart. So what can account for this slippage from common sense to conspiracy theory?

Obama made the story of his increasing commitment to Christianity a central tenet of his campaign character narrative; whole chapters were devoted to matters of faith in his book "The Audacity of Hope." But in the radioactive wake of the Rev. Jeremiah Wright, Obama left his longstanding church and has not yet found a new congregation.

The absence of Obama attending Sunday services regularly has been a repeated "just askin'..." talking point on right wing news and radio -- and indeed, the Pew poll shows that most respondents got their "views" about Obama's religion from "the media."

The meticulous attention that some liberals paid to George Bush's golf game is now directed at Obama hitting the links -- with the added subtext that he's playing when he should be praying. In the eyes of his opponents, Obama is an elitist silently laughing at those bitter Americans who cling to their guns and religion. The fact that an icon of the religious right like Ronald Reagan rarely attended Sunday services is ignored.

But these poll numbers are also an ugly proxy for the campaign to convince the American people that their president is somehow "other" -- an "Alien in the White House," as a Wall Street Journal editorial page headline called him recently.

The birther myths build off this sense of Obama as a sinister fraud, casting Obama as not just anti-American in his beliefs but actually un-American and therefore not constitutionally eligible to be president.

These conspiracy theories in turn build off chain e-mails that have been circulating since at least 2008, portraying Obama as a Muslim Marxist Manchurian Candidate determined to undermine the Republic from within -- a fright wing fantasy that has proliferated via the internet. Depressingly, a CNN poll earlier this month found that 27 percent of all Americans believe that Obama was either definitely or probably not born in the USA -- a belief shared by 41 percent of Republicans.

"Everyone is entitled to their own opinion," the late Sen. Daniel Patrick Moynihan famously said, "but not their own facts." But the rise of partisan media means that Americans are self-segregating themselves into separate political realities where we can't even agree on basic facts like the president's religion and birthplace.

While no responsible Republican would ever explicitly encourage these beliefs, there are plenty who delight in these poll findings because it presumably points to partisan gain. Truth is not as important as the pursuit of power in professional partisans' eyes, and dividing to conquer is a time-honored trick.

Each president since at least Nixon has faced a concerted effort to de-legitimize the duly elected president of the opposing party from day one. But with Barack Obama, the fact that he is the first African-American president with a middle name Hussein has led to a new level of ugliness and suspicion. Attacks that demonize the president are poisoning the civic well from which we all drink.

The opinions expressed in this commentary are solely those of John Avlon.