In a fundraising email sent on Friday, National Organization for Marriage (NOM) President Brian Brown criticized presidential hopeful Mike Buttigieg's marriage as illegitimate.

Buttigieg, the openly gay mayor of South Bend, Indiana, has in recent weeks criticized Vice President Mike Pence's opposition to LGBT rights, including marriage equality.

One speech in particular seemed to hit a nerve. After praising his marriage to Chasten – “And yes Mr. Vice President, it has moved me closer to God” – Buttigieg, speaking to Pence, added: “If you got a problem with who I am, your problem is not with me – your quarrel, sir, is with my creator.”

In several interviews, Pence responded that Buttigieg “should know better” than to criticize his Christian faith.

(Related: Pete Buttigieg: Mike Pence shouldn't use religion to justify harming LGBT community.)

Brown said that Buttigieg had “slapped all of us [in the face] who believe in the sanctity of marriage as the union of a man and a woman.”

“Mr. Mayor, let me firstly say that we don’t have a problem with ‘who you are;’ we have a problem with what you believe and your insistence that your beliefs be imposed on the rest of society,” Brown wrote. “Secondly, your Creator did not make you gay. Supporters of marriage, like Vice President Pence, are not out campaigning against gays and lesbians for ‘who they are.’ Our cause is not condemning homosexuals, it’s upholding marriage. NOM has always maintained that gays and lesbians are free to live their lifestyle as they choose, but they are not free to redefine marriage for all of society.”

Writing at Right Wing Watch, Peter Montgomery called Brown's assertion “laughable.”

Brown has “supported laws that impose long criminal sentences for consensual homosexual conduct” and has “stood side by side with – and lavished praise on – authoritarian leaders who restrict the lives and freedom of LGBT people,” Montgomery wrote.

Brown also characterized Buttigieg's marriage as illegitimate – “That relationship is not marriage, and no judicial decree or political act can ever make it so.” – and vowed to overturn the Supreme Court's 2015 ruling in Obergefell that struck down state laws and constitutional amendments that restrict marriage to heterosexual unions.

Buttigieg is expected to formally announce his presidential campaign on Sunday.