The following correction was printed in the Guardian's Corrections and clarifications column, Wednesday September 10 2008

In the article below it was stated that Sarah Palin was a member of the Alaskan Independence party. The Alaskan Independence party subsequently issued a statement saying it had been wrong to claim Sarah Palin was a member.

New revelations about the Republican vice-presidential candidate Sarah Palin — including her membership of a party that wants Alaskans to vote on becoming a separate country — are raising questions about how thoroughly John McCain's campaign vetted her background before adding her to the ticket.

Palin was a member of the Alaskan Independence party (AIP) before becoming an elected Republican official, according to party members, and recorded a video message for the AIP convention this year. The AIP's chief goal is securing Alaska a vote on seceding from the US, a goal that party leaders believe the state was denied before it became part of the US almost 50 years ago.

Yet it is the AIP's motto, "Alaska First, Alaska Always", that may cause the most trouble for McCain. The Republican's campaign slogan this year is "Country First".

At the convention where Palin's video was played, the AIP vice-chairman, George Clark, told the audience that she was an AIP member before getting her first political post as mayor of the small town of Wasilla, Alaska.

"But you get along to go along — she eventually joined the Republican party, where she had all kinds of problems with their ethics, and well, I won't go into that," Clark said. "She also had about an 80% approval rating, and is pretty well sympathetic to her former membership."

Palin suggested in a July interview with CNBC news that she would insist on making Alaskan issues a high priority before agreeing to serve as a vice-presidential candidate. "We want to make sure that that VP slot would be a fruitful type of position, especially for Alaskans, and for the things we're trying to accomplish up here for the rest of the US, before I can even start addressing that question," she said.

In response to the AIP flap, the McCain camp denied that Palin was a party member and released voter registration documents that showed her affiliating with Republicans. "If the Alaska Independence Party at some point taught Governor Palin their secret handshake, there is no record of it," McCain aide Michael Goldfarb wrote on the campaign's website. "Otherwise, the only relevant criterion for membership in a party is registration -- and Palin has never been a member of the AIP.

Intense media scrutiny of Palin since she became McCain's running mate four days ago has led to speculation that the Republican party failed to fully examine her background. In addition to the pregnancy of Palin's 17-year-old unmarried daughter, Bristol, several other disclosures threaten to throw the McCain camp into turmoil.

Palin has promoted her independence from Alaska's powerful senior senator, Ted Stevens, who is facing seven criminal charges in Washington. But she served for two years as a director for one of his political groups that was able to raise unlimited money from corporate patrons.

Palin faced pressure to resign as mayor of Wasilla in 1997 after she fired the city police chief for not fully supporting her agenda, leading to a lawsuit for breach of contract.

In Alaska, Palin faces an ethics investigation into whether she abused her office by firing the public safety commissioner, who refused to intervene in a messy divorce case involving her sister. Palin has hired an attorney to help her handle the case, leading to another round of embarrassing press coverage.

McCain's spokesman, Tucker Eskew, defended the selection: "This legal defence is neither new nor uncommon nor at all political. It is a matter of her job and is not recent and it is not related to her selection on the McCain-Palin ticket."