Of all the ways that Congress is different today from what it was 50 years ago, this is one of the most basic: Population shifts, both from movement by citizens and growing numbers of international immigrants, have led to a huge change in the regional balance of power in the House of Representatives.

Over the last half century, the South and the Pacific coast have gained the most. Taken together, the two regions have seen a net gain of more than 50 seats. By contrast, the Mid-Atlantic and the Midwest have been big losers, dropping a combined 44 seats over the same time period.

These patterns become even clearer when one looks at individual states. California, of course, has been the biggest beneficiary of reapportionment over the last five decades: Since the 1950s, the Golden State has picked up 23 seats all by itself. In the South, Florida and Texas have been big gainers as well, picking up 17 and 10 seats, respectively.

Among states that have lost ground, New York and Pennsylvania have taken the greatest hit, losing 14 and 11 seats in their respective delegations. Declines in the Midwest have been more diffused; every state in the region has dropped seats, but no single state has been hit especially hard. Illinois and Ohio tied for the greatest net reduction, losing five seats apiece.

A number of reasons account for these patterns. Michael Barone, author of The Almanac of American Politics Since the Early 1970s, emphasizes the changing nature of the economy.

“You’ve had people shift out of the Northern and Great Lakes states to various parts of the Southeast because of more vibrant economies in those areas,” Barone said. “In the South the development of air-conditioning and the end of segregation gave people a greater opportunity.”

Barone cited other economic factors as well. “Many people would argue there is growth from high tax to low tax, from union to non-union,” he said. “That has been the trend.”

Rep. Tom Davis (R-Va.), another student of Congressional districts and a one-time head of the National Republican Congressional Committee, echoes Barone’s theory.

“There are a lot of reasons. First of all, the labor base has moved south and west because it’s cheaper to do things in those regions. You don’t have the regulatory and the tax problems in the Northeast,” Davis said. “Where business goes, population goes. Take a look at the work climate. Why has Utah grown so quickly? Or Nevada?”

The military, especially during the Cold War, also helped economic growth in the South, Davis said. “The defense buildup has fueled the Southern economy,” he said.

Davis also noted the graying of the population in the Northeast. “Seniors, when they retire, like to go to warm weather,” Davis explains. “You see it in North Carolina, Florida and Arizona.”

In addition to migration between the states, international immigration has played a major role in population shifts. It is not a coincidence that states with high percentages of immigrants have gained a growing share of seats.

Even though many citizens leave California every year, its population continues to soar.

“California is a little bit of an outlier. Although it’s gained a lot of immigrants, over the last 20 years native Americans are emigrating,” at least domestically, explained William Frey, visiting fellow at the Brookings Institution Metropolitan Policy Program adding that “Texas and Florida are gaining in both populations.”

Immigration, both legal and illegal, has caused a noticeable shift in the population balance. In a 2003 study, the Center for Immigration Studies noted that the most recent Census indicates “that almost seven million illegal aliens were counted,” leading to a “significant erosion of the political influence” of low-immigration states.

For example, in the 2000 reapportionment alone, there was a net shift of nine seats from states with low rates of non-citizens to states with higher rates.

Barone, for his part, pointed to a portion of Los Angeles as an example of an area that has been shaped to a large degree by immigration.

“Look at the 31st district of California,” Barone singles out. “It doesn’t have very many voters because the people there aren’t citizens.”

Of course, the change in demographics has had a profound influence on the nation’s partisan balance. The South, once Democratic despite its conservatism, is now solidly Republican. And with more seats leaning in its direction, conservatives have gained ground nationally in Congress.

“In the past, you had the anomaly that the South was still voting against” Union Gen. William T. Sherman, 96 years after the Civil War, Barone explained. “They aren’t voting against Sherman anymore.” The elimination of the Northern/Southern split in the Democratic party has also led to a homogenization of the political parties.

“We’ve come into a period where parties are more coherent, and less diverse — where most members stand for what the national party stands for,” Barone says. “In the 1990s, we saw a move to more straight party-line votes.”

Davis adds that “you used to have debate where you had southern Democrats, northern Democrats, and Republicans. It has been more aligned” in recent years. “The South is still very much on top. They dominated when Democrats controlled and they dominate now with Republicans in power.”

Davis reminds observers, though, that Congressional representation is dynamic.

“For every action, there’s a reaction. There are shifts back and forth. The party that stays ahead of the curve is the one that ends up dominating.”

In other words, check back again in another 50 years.