States projected to gain and lose Congressional seats in the next Census, according to mid-decade population estimates. (Source: Election Data Services Inc.)

Continued population migration to the South and West could give Republicans an electoral leg up after the decennial reapportionment process, according to preliminary Census Bureau figures.

Annual estimates of population growth show red states gaining residents at a faster rate than blue states, while traditionally blue states in the Northeast continue a decades-long population decline.

According to data compiled by Election Data Services, a nonpartisan tracker, current trends indicate Texas would likely gain three additional House seats after the 2020 Census, growing its Congressional delegation from 36 to 39.

Reliably red Arizona would add another seat, based on current trends, increasing its delegation to 10 House members. Blue states California and Oregon are both projected to gain seats, as are swing states Colorado, Florida, North Carolina and Virginia.

But states most likely to lose seats are almost all reliably Democratic: Illinois, Michigan, Minnesota, New York, Pennsylvania and Rhode Island — all states Democratic presidential candidates have carried in the last six national elections — are projected to lose enough population to shed a seat.

Swing Ohio is projected to drop a seat, and Alabama and West Virginia, both states Republicans have won in the last four national elections, would also lose seats.

That means blue states would lose a net of four electoral votes, and red states would gain a net of two, a shift of six total electoral votes, the equivalent of flipping a state the size of Iowa from the blue column to the red column.

“While Democrats are making inroads in the South and Mountain West, these aren’t occurring fast enough to make up for apportionment losses due to population shifts,” said William Frey, a demographer at the Brookings Institute. “Texas continues to be the mega migration magnet for the nation, and, if that continues, the Republicans have a strong card in their deck in the future.”

The Bureau’s estimates are preliminary, and population trends could easily shift in the six years before the 2020 Census. Before the 2010 Census, the economic recession dramatically slowed down migration patterns and prevented some states from losing seats.

“We are only at the midpoint of the decade, and a lot of things could change before the next Census is taken in 2020,” said Kimball Brace, president of Election Data Services. “It’s important to remember that major events like [Hurricane] Katrina and the 2008 recession each changed population growth patterns and that impacted and changed the next apportionment.”

But current projections show some states are just inches, population-wise, from retaining, gaining or losing seats. Rhode Island is a little more than 21,000 residents from falling below the threshold they need to keep their second Congressional seat, which the state has maintained since losing its third seat after the 1930 Census.

Oregon must add 53,000 residents to gain their sixth district. One projection shows Virginia gaining a seat by the narrowest of margins, about 14,000 residents — about two-tenths of the state’s population.

The U.S. Constitution grants every state at least one member of the House of Representatives, and additional members based on population. Demographers are able to use Census data to project which state would get which seat in Congress — for example, if Congress were made up of just 51 members, California, the nation’s largest state by population, would be awarded the 51st seat.

Under the current projections, Virginia would be awarded the 435th seat. Alabama would just miss out: They would keep their seventh seat if Congress expanded to 436 members.

The smallest state, Wyoming, would only be awarded a second seat if Congress expanded to 746 members. Rep. Cynthia Lummis (R) represents fewer constituents, 613,606, than any other member of Congress, according to current Census population estimates.

Montana Rep. Ryan Zinke (R) still represents the largest number of constituents, at just under 1.1 million. Montana would gain a second member of Congress if the House expanded by just four seats, to 439 members.

The long-term reapportionment trend shows greater growth in Southern and Western states over a century. California has gained seats in every decennial reapportionment process since 1860, except in the 2010 cycle. Texas has gained at least one seat every decade since 1950.

On the other hand, New York’s Congressional delegation has shrunk precipitously, from a peak of 45 seats after the 1930 Census to just 27 after reapportionment in 2010. Ohio’s delegation has plummeted from 24 seats after 1930 to just 16 today, and Pennsylvania’s has been cut in half, from 36 seats after the 1910 Census to 18 today.

See the historical size of each state’s delegation here, courtesy EDS.