WASHINGTON — Two of the country’s largest and most historic Episcopal congregations — both in Fairfax, Va.— will vote next week on whether to leave the American church on ideological grounds and affiliate instead with a Nigerian archbishop. The decision could lead to a bitter court battle and the loss of $25 million in property.

Many members of the Falls Church and Truro Church, as well as some conservative leaders around the country, hope a split will establish a legal structure that would make it easier for more like-minded congregations to depart the national denomination.

Some conservatives in the Episcopal Church, the American wing of the worldwide Anglican Communion, believe the church abandoned scripture by installing a gay bishop in New Hampshire in 2003, among other things. Those feelings of alienation were strengthened when Bishop Katharine Jefferts Schori — who supports the New Hampshire bishop — was elected this summer to lead the national church.

Three other churches in the 193-congregation Virginia diocese — the nation’s largest — are also voting this month.

While some orthodox congregations have been leaving since 2003 — as well as in the 1970s, when ordinations of women began — advocates think they are getting closer to creating a new, American-based umbrella organization that would essentially compete with the 226-year-old Episcopal Church.

“In one sense there is a sadness because this feels like a death,” said Mary Springmann, who worships at Truro and plans to vote to split when a week of voting begins December 10. “Like someone who has been gravely ill for a long time, you keep hoping there’s going to be a recovery. And at some point you realize it’s not going to happen. Right now … there is a feeling of hope and expectancy about where God is going to lead us next.”

If the votes at the Falls Church and Truro succeed, as their leaders predict, the 3,000 active members of the two churches would join a new, Fairfax-based organization that answers to Nigerian Archbishop Peter Akinola, leader of the 17 million member Nigerian church and an advocate of jailing gays. The new group hopes to become an American-based denomination for orthodox Episcopalians.

The Falls Church and Truro alone are worth more than $25 million in real estate, according to county records, not to mention the powerful sentimental value of churches formed in the 1700s — before the American denomination existed — when they were still part of the Church of England. George Washington was a member of the vestry at The Falls Church.

The Fairfax-based Convocation for Anglicans in North America attracted attention this summer when Rector Martyn Minns of Truro started a Virginia-based “mission” of the Episcopal Church of Nigeria at CANA — a twist on the typical missionary paradigm.