Opinion | We analyzed 3.7 million election ads. Here’s what we learned.

With eight days to go until the election, Democrats face the very real possibility that they won’t win the presidency, Senate or House. But if the party does come up short, it won’t be for lack of trying — at least when it comes to advertising.

To figure out which party won this election’s ad war, Times Opinion dug into data on 3.7 million political ad airings on broadcast TV from Aug. 1 to Oct. 25. We combined those records, provided by AdImpact, with TV viewership estimates from Comscore to calculate how many households saw each ad.

The data is clear: Democrats dominated the airwaves.

Between August and late October, Democratic campaigns and PACs poured $213 million more into TV broadcast advertising than their Republican opponents. The extra spending secured Democrats a lead of 9.9 billion ad views nationwide.

In the presidential race, Kamala Harris outpaced Donald Trump, racking up 11.8 billion ad views to Mr. Trump’s 8.5 billion. Though her margins were small, Ms. Harris took the lead in ad viewership across five of the seven swing states. And she notched sizable victories in less competitive states, outspending Mr. Trump nine to one on nationwide ad buys.

Each campaign is most likely keeping an eye on the other’s spending to make sure they don’t fall too far behind in the election’s final stretch.

“It’s an arms race. If Harris buys 100 ads today in Philadelphia, Trump wants to counter that,” said Lynn Vavreck, a political scientist at U.C.L.A. “That’s why those poor people in Pennsylvania keep seeing more and more ads. You definitely don’t want to cede the endgame to your opponent.”

(Poor Pennsylvanians, indeed. Our analysis found that the state’s households have seen an average of 1,391 political ads since Aug. 1 — about 16 per day.)

Ms. Harris and her allies also outspent Mr. Trump six to one on radio ads and three to one on Google and Meta between early- and mid-October, according to the Wesleyan Media Project.

Looking at the nine most competitive Senate races, Republican candidates struggled. They had the ad advantage in just three races — Montana, Ohio and Pennsylvania — and by slim margins at that.

The Democrats’ biggest advantage was in Arizona, where households watched, on average, 160 more ads for the party’s candidate, Ruben Gallego, than they did for the Republican Kari Lake. Democrats were also competitive in Texas, where Senator Ted Cruz trailed the Democratic challenger Colin Allred by 92 ad views per household.

If Republicans win enough seats next week to gain a majority in the Senate, then there’s a good chance that Donald Trump will win too, given how many competitive Senate races are in swing states. That means the House could be the Democrats’ best chance at checking a Republican legislative agenda in 2025.

Erika Franklin Fowler, a political science professor at Wesleyan University and director of the Wesleyan Media Project, told me that advertising may be more effective for House candidates because they have a bigger opportunity to define themselves.

“Often, advertising is the only way that voters will hear about those candidates,” Fowler said.

That’s good news for Democrats, as they had the most ad views in all 25 House races that Cook Political Report rated as tossups.

According to a 2021 study by Vavreck, John Sides and Christopher Warshaw, House candidates who aired 100 more ads than their opponent added about 0.08 percentage points to their final vote margins. The same advertising lead gave Senate candidates a 0.04 to 0.06 point bump and presidential candidates just 0.02 points.

“We have fairly decent evidence that TV advertising has a very small but non-zero effect,” said David Broockman, a political scientist at Berkeley. “And when you multiply a very small effect by a gigantic amount of money, it could swing enough votes to change the outcome in a close election.”

The effectiveness of the Democrats’ advertising across House races may come down to one thing: how much voters care about abortion rights. Democratic ads about reproductive rights received more views than any other topic in 42 of the 99 congressional races in which the party advertised.

If abortion mobilizes Democratic voters the way it did in the 2022 midterms, Ms. Harris will also benefit. After taxation, reproductive rights were her and her allies’ top advertising priority. In the run-up to the election, her campaign continues to emphasize the issue.

While Ms. Harris has run a smattering of ads about taking on drug cartels at the border, Mr. Trump and down-ballot Republican candidates have made immigration their focus. They spent $415 million on ads that mentioned it to Democrats’ $104 million.

Many of the Republicans’ ads linked immigration and crime, blaming Ms. Harris and Democrats for a handful of grisly attacks committed by undocumented immigrants. (The ads ignore the fact that U.S. immigrants are less likely to commit crimes than native-born Americans.)

It’s a battle-tested strategy for Republicans, who are riffing on the racist Willie Horton ad that helped George H.W. Bush win in 1988. Mr. Trump has campaigned as the law and order candidate before; one of his ads this year, featuring a woman whose son was tragically murdered by an undocumented immigrant, is largely a retread of a spot he ran in 2016.

According to the final Times/Siena national poll, immigration and abortion were tied as the most important issue to likely voters. If the former mobilizes more people to vote than the latter, Harris could be in trouble.

Advertising volume is important, but it doesn’t guarantee victory. Hillary Clinton lost in 2016 despite airing more than twice as many ads as Mr. Trump in the final nine weeks of the election. If the Republicans win next week, Democrats may need to rethink how they reach out to voters.