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A Data curation
A diachronic corpus is required to measure and analyze polarization over time (DiMaggio et al., 1996). We collect and annotate data across a long period to address the issue of distributional shifts across years (Desai et al., 2019; Rijhwani and Preotiuc-Pietro, 2020; Bender et al., 2021) and help build robust models that can generalize beyond certain periods.
Additionally, the raw data on top of which we annotate needs to satisfy the following constraints: (1) for human annotation to be tractable, the articles should share some level of topical coherence; (2) for the data to be useful for the larger community, the content should also cover a range of common discussions in politics across the aisle; and (3) the articles should come from a consistent set of news outlets, forming a continuous and ideologically balanced corpus.
We start with the diachronic corpus of political news articles of Desai et al. (2019) which covers years 1922-1986, the longest-spanning dataset to our knowledge. This corpus is a subset of news articles from the Corpus of Historical American English (COHA, Davies (2012)). To extract topically coherent articles, we investigate the topics and articles across multiple LDA (Blei et al., 2003) runs varying the number of topics (15, 20, 30, 50), aiming to arrive at a cluster of topics that share common points of discussion and collectively will yield a sizable number of articles each year from the same news outlets.
The LDA models consistently showed one prominent topic—the federal budget—across 5 news outlets with balanced ideology (c.f. Table 8): Chicago Tribune (CT), Wall Street Journal (WSJ), Christian Science Monitor (CSM), the New York Times (NYT), and Time Magazine (TM). Because federal budget stories touch on all aspects of the federal activity, this topic appeals to both liberal and conservative media and thus can provide a good testing ground to showcase our proposed ideological annotation method. In addition to the core federal budget topic (topic 5 of Table 7), we also include other topics such as health and education that are integral parts of ideological beliefs in the United States, and when discussed at the federal government level, are typically related to the federal budget. The top vocabulary of the cluster is shown in Table 7. In an effort to purge articles unrelated to the federal budget, we selected
Topic1:
Trade	bank, market, farm, loan, export, agricul-
tur, farmer, dollar, food, debt
Topic2:
Business	incom, tax, revenu, profit, corpor, financ,
treasuri, pay, sale, bond
Topic3:
Education	school, univers, educ, student, colleg, pro-
fessor, institut, teacher, research, graduat
Topic4:
Defense	nuclear, missil, weapon, atom, test, energi,
strateg, bomb, space, pentagon
Topic5:
Economy	budget, billion, economi, inflat, economic,
deficit, unemployment, cut, dollar, rate
Topic6:
Health/Race	negro, hospit, medic, health, racial, south-
ern, discrimin, doctor, contra, black
Topic7:
Industry	compani, contract, plant, steel, coal, wage,
railroad, corpor, manufactur, miner
only those that contain words such as "federal" and "congress", and excluded those that mention state budget, and letters to editors. (Note that during annotation, we also discard articles that are unrelated to the federal budget.) After this curation, the total number of articles is 5,706 from the 5 outlets.
To account for the sparsity of articles in the first decades and their density in later decades, we narrowed down the articles to the period from 1947 to 1974. We believe this period is fitting because it includes various ideological combinations of the tripartite composition of the American government, Congress and presidency.[10] The total number of articles in the final corpus of political articles on the federal budget from 1947 to 1974 is 1,749.
B Proclaimed ideology of news outlets
Table 7: Top words from topics selected in our cluster, from the 50-topic LDA model that yielded the most well-delimited topics.
Adfontes	Allsides	MBFC	Average
CSM	-.06	0.00	-.16	-.07
CT	-.04	NA	.34	.15
NYT	-.20	-.5	-.4	-.36
TM	-.10	-.5	-.6	-.4
WSJ	.15	.25	.58	.32
Table 8: Ideological bias of news outlets from common references of media bias. We use the average in our analyses.
<sup>10</sup>For example, between 1947-49, Congress was Republican and the President was a Democrat while the story flipped between 1955-57.