Republicans in congressional districts with low percentages of married couples face the strongest challenges this fall as Democrats seek to regain control of the House of Representatives.

Republicans dominate districts that have large percentages of married people. Democratic districts are stacked with people who have never married.

This "marriage gap" is the biggest demographic difference that separates Republican congressional districts from Democratic ones, according to a USA Today analysis of 2005 Census Bureau data released last month.

Democrats must gain 15 seats in the election Nov. 7 to take control of the House. More than two-thirds of vulnerable Republican districts have fewer married people than found in traditional GOP districts.

Four out of five Republicans in districts with the party's lowest marriage rates are in trouble. The nonpartisan National Journal ranks Arizona's 8th District and Colorado's 7th District as the Republican seats most in jeopardy. These districts, held by Republicans not seeking re-election, rank 251st and 307th respectively in marriage rates among the 435 districts.

The wedding ring is crucial in this fall's elections because married people are the core of the Republican base. A USA Today analysis of Census data found:

Republicans control 49 of the 50 districts with the highest rates of married people. The exception: Rep. Rush Holt, D-N.J., who ranks 32nd and represents the affluent area around Princeton University.

Democrats represent all 50 districts that have the highest rates of adults who have never married and 96 of the top 100.

The parties have similar shares of divorced and widowed people in their districts.

The political tug-of-war is between people who are married and those who have never been.

This "never married" group covers a variety of groups who form the Democratic base: young people, those who marry late in life, single parents, gays and heterosexuals who live together.

The marriage divide has drawn attention in recent presidential races. President Bush beat John Kerry by 15 percentage points among married people in 2004 and lost by 18 percentage points among unmarried people, according to an exit poll conducted by national news media organizations.

Rep. John Linder, R-Ga., has the district with the highest marriage rate (66.1 percent). Linder, who represents Atlanta suburbs, says the marriage gap is caused by how values change when people marry. "People get more conservative when they settle down," he says.

Democratic pollster Mark Mellman says the marriage gap is magnified because married people vote in greater numbers than unmarried people. "We have to appeal more to married people, particularly married women," he says. "There's no reason we should lose those votes, but we have."