Shoppers in downtown Washington. (Agence France-Presse via Getty Images)

Last year the United States had the lowest rate of population growth of any year since the Great Depression, according to census figures released Tuesday.

The milestone is due largely to the aging of the population, with more deaths last year than at any time since 2000, according to William Frey, a demographer and senior fellow at the Brookings Institution.

The nation grew by 0.695 percent between 2015 and 2016 to 323.1 million, down from 0.732 percent the previous year — the lowest increase since 1937-1938, when it was 0.60 percent.

Immigration also declined, though for the past three years it has been higher than it had been since before the recession of 2007-2009. But the fall in the natural increase, from 4.07 to 3.84 per 1,000, reflecting fewer births and more deaths, is the lead cause of the slowdown — and the trend is expected to continue, Frey said.

“The aging of the population is the main thing,” he said. “We still have a positive natural increase, and there are other countries that don’t have that” — such as Germany and Japan.

But in coming years the increase will continue to decline, with serious policy implications, he said. “We need to pay attention to the dependent older population who’s going to have to be taken care of, through Social Security and Medicare and general support for them.”

At the same time, he cautioned that the United States will need to invest in immigrants who are helping to shore up the younger segment of the labor force.

The latest numbers show some states being hit harder by population loss while others are on an upswing — shifts that could affect future statewide and national elections.

Western and Southern states such as Nevada, Arizona and Florida, which took big hits during and after the recession, have been growing recently, while states with higher costs of living, including New York and California, and Midwestern states such as Ohio and Illinois, have seen a decline in growth as people have moved away.

Outmigration from California had stagnated during the recession and post-recession period, but now the Golden State appears to be losing more migrants to neighboring states, in a phenomenon known as domestic outmigration, Frey said. A similar pattern is occurring between New York and Southeastern states.

Utah is now the fastest-growing state — its population increased 2 percent to 3.1 million — while North Dakota, the growth leader in 2014-2015, fell to become the 15th-slowest-growing state as its oil extraction economy withered. Illinois leads the nation in population losses for the third year in a row, with its largest domestic outmigration since 1990.

Texas had the highest numeric gains, with growth in immigration, domestic migration and natural increase. Florida, which bled population during and after the recession, also ranks high because of immigration and domestic migration. California and New York, on the other hand, rank high on immigration and natural increase but are among the nation’s biggest losers in terms of domestic migration, ranking 49th and 51st, respectively.

The District of Columbia registered its highest population count since the 1970s, at 681,170 — an upward trend that is expected to continue.

The changes have important implications for future elections. Projecting current trends onto the 2020 Census, Frey calculated that Texas would gain three electoral college votes, Florida would gain two, and Arizona, Colorado, North Carolina and Oregon would gain one each. Alabama, Illinois, Michigan, Minnesota, Ohio, Pennsylvania, Rhode Island and West Virginia would lose one apiece. Given this November’s voting, the shifts would result in Donald Trump netting two additional electoral college votes from Hillary Clinton.

The changes imply that a lot more states could be competitive in national elections. Typically the Midwest and the Northeast have voted for Democrats while Southern and Southwestern states have voted for Republicans. But Barack Obama won some traditionally Republican states and Trump picked up some traditionally Democratic ones.

As domestic migration continues, Republicans will no longer be able to rely on wins in Southern states and Democrats will have to play stronger defense in northern industrial areas they once took for granted, Frey said, adding, “It’s kind of up for grabs right now.”

Outmigration from areas with declining economies can create a vicious cycle, said Philip Cohen, a sociology professor at the University of Maryland. “When the good prospects are elsewhere, people with good prospects leave,” he said, adding, “The middle of the country is still hollowing out overall in the long-term.”

At the same time, the recent election showed the extent to which small towns in rural areas are still in play, Cohen said. If future population shifts make once-reliably red or blue states competitive, “populations of small towns could become very important.”

And as states such as California see more outmigration, immigrants from Mexico and Central America could increasingly head directly to states such as North Carolina and Iowa where there are jobs. Once they and their children become citizens, this could have electoral implications.

“In the small towns where immigrants are going, they can have a big effect,” Cohen said.

Scott Clement contributed to this report.