In 2008, Barack Obama ran as the Democratic Presidential nominee against John McCain, the Republican Presidential nominee. To be eligible for the post, one must be a "natural born citizen." Several prominent political figures in the United States, who are naturalized citizens of the United States, are not eligible for this post. The 14th Amendment to the United States Constitution establishes that anyone born in the United States, regardless of paternity (with the now spent exception of Indians not taxed), is a citizen of the United States.
Obama was born in Hawaii, at at time when it was a U.S. state, to a U.S. citizen mother and a Kenyan father. McCain was born in the Panama Canal Territory, which was not part of any U.S. state. The legal arguments that McCain might not be a natural born citizen of the United States are more weighty.
Yet, in many states, there are still more Republicans who believe that Obama is not a natural born citizen of the United States than there are who don't doubt Obama's citizenship. This was an issue well before the election was over in conservative circles. Partisans can believe odd things. Conservative intellectuals and leaders, who know better, are content to play demagogue, rather than correct the error that is widespread in their base.
But, there is more to it than that. While knowledge of McCain's citizenship issue was widespread, appearing in major newspapers and widely known to the liberal equivalent of the hard core political activists who churned up the "birther" movement among conservatives, the issue never gained traction with liberals. Liberal opinion leaders considered the issue, acknowledged the ambiguity, and determined that it wasn't appropriate to make the case against McCain on these grounds.
Two factors were at play, and both mattered.
First, a campaign smear aimed at showing that John McCain was unAmerican had little voter appeal. The white, senior citizen, long time U.S. Senator, former war hero fit what the conservative and independent voters that he sought felt was an American, regardless of the facts, so the issue had no emotional appeal.
Moreover, conservatives, generally, are uncomfortable with the notion of birthright citizenship, as opposed to paternity based citizenship, something often expressed by their frequent mischaracterization of the U.S. born children of illegal aliens as illegal aliens. Distaste for the 14th Amendment isn't too surprising. Today's birthers are the political descendants of those who favored secession to defend the institution of slavery, and late sought to protect the Southern segregation system from the Civil Rights movement. The 14th Amendment's birthright citizenship provision was enacted for the express purpose of undermining them politically.
In contrast, for conservatives, the fear that Obama (an adult convert to Christianity who had a secular upbringing) was a secret Muslim were real, and they distrusted his black skin, the fact that he had a Kenyan Muslim father, and the fact that Obama is not culturally coming from the same place as most African Americans despite looking the part. The birther claim is baseless, but has emotional resonance for xenophobes who really care about the "natural born citizen" requirement because they want to keep out Presidential candidates who are culturally different from them, without regard to the actual rules for determining who is a natural born citizen.
Second, liberals value rule of law, fairness, civility and honesty in political discourse. Conservatives don't. A birther smear goes against a typical liberal's ethical framework, and so it doesn't sell to liberals. Conservatives, in contrast, don't understand what is wrong with lying about an opposing Presidential candidate's personal life in an effort to prevent him from being elected. They care more about the result than the process.
Liberals think that politicians should behave like cricketers. Conservatives think that politics is war by other means.
Unbossed argues, and I agree, that the media should confront and run stories about prominent people who wantonly distribute misinformation about public issues.