WASHINGTON — Their rout complete, Republicans are celebrating sweeping victories from North Carolina to Colorado.

The GOP’s success, however, may have as much to do with the dynamics of a low-turnout midterm election as widespread dissatisfaction with President Barack Obama’s leadership, according to operatives and pollsters still working to understand how the Republican Party exceeded its own high expectations this week.

An estimated 37 percent of eligible voters cast a ballot in the midterm elections, said Michael McDonald, an associate professor at the University of Florida. If that projection holds, it would be the lowest turnout since 1942, when Americans busy fighting a war let the level drop to 34 percent.

This year’s electorate was largely older, whiter and more male than is often the case in presidential years. Republican pollsters knew the GOP would benefit from a smaller and less diverse electorate, but didn’t predict the extent to which some Democratic allies would stay home on Election Day.

“The composition of the electorate was fundamentally different than anyone predicted,” said Gene Ulm, a pollster at Public Opinion Strategies, a firm that worked for Republican candidates or super PACs in Maine, Iowa, Colorado, New Mexico, North Carolina and Kentucky, among other states.

Indeed, the GOP scored sweeping victories across the rural and Southern states that Republicans typically dominate. But their success went much further, extending into battleground states and regions that are solidly Democratic.

A closer look at the electorate offers some clues about how the GOP seized the Senate majority, expanded its House majority and broadened its advantage in governors’ mansions across the nation.

Nationally, 6 in 10 voters were dissatisfied or angry with the Obama administration, and 6 in 10 separately said the same of Republicans in Congress, according to exit polls conducted for the AP and the television networks by Edison Research.