Barack Obama told a San Francisco-based Lesbian Gay Bisexual Transgender group on Sunday that he opposes a California ballot measure that would ban same-sex marriage.

Prop 8 was added to the November ballot after the state supreme court ruled that a previous voter initiative barring gay marriage was unconstitutional.

“I am proud to join with and support the LGBT community in an effort to set our nation on a course that recognizes LGBT Americans with full equality under the law,” reads the Obama letter, available on the Alice B. Toklas LGBT Democratic Club Web site.

Obama said he supports "extending fully equal rights and benefits to same sex couples under both state and federal law,” and called the measure a "divisive and discriminatory" effort to amend California's constitution.

In response, John McCain's campaign told The Sacramento Bee that Obama is guilty of flip-flopping, saying his support for the measure contradicts an earlier statement in which the Democratic candidate said it should be left to the states to decide their marriage policies.

Obama has rejected the military's "don't-ask-don't-tell" policy on keeping sexual orientation quiet and supports a repeal of the Defense of Marriage Act. Campaign spokesman Ben LaBolt told FOXNews.com that Obama also opposes efforts to define marriage within a constitutional framework.

“Senator Obama supports civil unions, and he has consistently opposed federal and state constitutional marriage amendments because, as we have seen in some states, enshrining a definition of marriage into the constitution can allow states to roll back the civil rights and benefits that are provided in domestic partnerships and civil unions,” LaBolt said.

McCain has said he supports the decision to let each state decide its own marriage laws, but backed a 2006 initiative in Arizona to define marriage as between a man and woman. It was defeated.

The Advocate newspaper reported Tuesday that the Log Cabin Republicans, composed of gay GOPers who support the California Supreme Court's ruling upholding the rights of gay couples to wed, confirmed that McCain had gone from neutral to supportive of the constitutional amendment, siding with conservative backers of the ballot measure.

"We obviously disagree with Senator McCain on this issue and do not believe he should have interjected himself into this state issue,” LCR President Patrick Sammon said in a statement. “Backing California’s ban sends the wrong signal to the independents who will decide this election because it creates the impression that he’s pandering to social conservative leaders.”

Randy Thomasson, of the Campaign for Children and Families, which introduced the measure, accused Obama of “pandering to San Francisco extremists,” according to The Sacramento Bee.

Click here to read Obama’s letter to the Alice B. Toklas LGBT Democratic Club.

Click here to read The Sacramento Bee article.

Click here to read The Advocate article.