House Democratic leader Nancy Pelosi, a Catholic mother of five from San Francisco, has fewer children in her district than any other member of Congress: 87,727.

Rep. Chris Cannon, R-Utah, an LDS father of eight, represents the most children: 278,398.

These two extremes reflect a stark demographic divide between the congressional districts controlled by the major political parties.

Republican House members overwhelmingly come from districts that have high percentages of married people and lots of children, according to a USA Today analysis of 2005 Census Bureau data released last month.

GOP Congress members represent 39.2 million children younger than 18, about 7 million more than Democrats. Republicans average 7,000 more children per district.

Many Democrats represent areas that have many single people and relatively few children. Democratic districts that have large numbers of children tend to be predominantly Hispanic or, to a lesser extent, African-American.

This "fertility gap" is crucial to understanding the differences between liberals and conservatives, says Arthur Brooks, a professor of public administration at Syracuse University. These childbearing patterns shape divisions over issues such as welfare, education and child tax credits, he says.

"Both sides are very pro-kids. They just express it in different ways," Brooks says. "Republicans are congenial to traditional families, which is clearly the best way for kids to grow up. But there are some kids who don't have that advantage, and Democrats are very concerned with helping those kids."

Children in Democratic districts are more likely to live in poverty and with single parents than kids in GOP districts.

Rep. Jose Serrano, D-N.Y., has 227,246 children in his Bronx district, the 10th most in the House. Only 29 percent of those children live with married parents.

By contrast, 84 percent of children in Cannon's central Utah district live with married parents.

"These numbers are amazing," Cannon says. "I see now where Jose is coming from."

Cannon used to have a locker next to Serrano at the congressional gym and considers him a friend. "The needs of kids in his district are just not the same as the needs of children in my district," Cannon says.

Marriage and parenthood define what's different about Democratic and Republican districts even more clearly than race, income, education or geography, USA TODAY's analysis of Census data found.

For example, Republicans represent seven of the 50 districts that have the highest concentration of blacks. Both parties are well represented among affluent and well-educated districts.