WASHINGTON — More than 150 Christian leaders, most of them conservative evangelicals and traditionalist Roman Catholics, issued a joint declaration today reaffirming their opposition to abortion and gay marriage and pledging to protect religious freedoms.

The 4,700-word document, called “The Manhattan Declaration: A Call of Christian Conscience,” sounds familiar themes from political and social debates over the health care overhaul and gay marriage battles.

While acknowledging that “Christians and our institutions have too often scandalously failed to uphold the institution of marriage,” the group rejects same-sex marriage. The declaration states that opening a legal door for gay marriage would do the same for “polyamorous partnerships, polygamous households, even adult brothers, sisters, or brothers and sisters living in incestuous relationships.”

President Barack Obama’s desire to reduce the need for abortion is “a commendable goal,” but his proposals are likely to increase the number of elective abortions, the document contends.

“The present administration is led and staffed by those who want to make abortions legal at any stage of fetal development, and who want to provide abortions at taxpayer expense,” it says.

Obama has said he wants to strike a balance on abortion coverage in the health care overhaul.

The declaration also cites threats to health care workers’ conscience clauses and anti-discrimination statutes it argues impinge on religious freedoms.

Signatories include 15 Roman Catholic bishops, including New York Archbishop Timothy Dolan and Washington Archbishop Donald Wuerl; Focus on the Family founder James Dobson; National Association of Evangelicals president Leith Anderson; seminary leaders, professors and pastors.