The Nation, the progressive magazine that has published since 1865, will publish monthly instead of every other week starting in January.

As part of the change, the magazine will now be a “bigger, richer” 84 pages, instead of the current 48 pages, Bhaskar Sunkara, the president of The Nation, said.

D.D. Guttenplan, The Nation’s editor, and Katrina vanden Heuvel, its editorial director, said that the publication would continue to focus on long-form analysis and news from the political left. Ms. vanden Heuvel said that the staff was reconsidering the role the print magazine plays alongside the brand’s other products, including its website, podcasts, events and a possible book imprint — and that the coverage in print had a long shelf life.

“People put aside magazines, circle what they want to read,” Ms. vanden Heuvel said.

Nearly all print publications face strong headwinds, including declining circulation and lower advertising revenue, as most readers turn to online products. Mr. Guttenplan said The Nation was in a slightly different situation because most of its subscribers get the print edition and print advertising is not a major source of revenue.