donald j. trump hasn<u+2019>t done much tv advertising. he prefers a different style of campaigning, with rallies and the free media coverage of them, along with his frequent twitter posts. fund-raising has taken a back seat because, as mr. trump said: <u+201c>i don<u+2019>t even know why i need so much money. why do i even need these commercials?<u+201d> with the election only a few weeks away, we<u+2019>re about to find out. since june, there have been roughly 300,000 tv commercials on behalf of presidential candidates. most of them have been in florida, ohio and north carolina, but a small number have been on national networks. according to data collected by kantar media/cmag, 80 percent of the ads to date were run by or on behalf of hillary clinton , while only 18 percent were shown by or on behalf of mr. trump. nearly everywhere the race is competitive, mrs. clinton has run far more ads. most presidential campaigns are not like this. they are more like a tug of war with both sides pulling equally hard. the flag in the middle usually doesn<u+2019>t move much, but that doesn<u+2019>t mean pulling hard is a waste of effort <u+2014> it<u+2019>s just hard to tell, because one side<u+2019>s effort cancels out the other<u+2019>s. but this year, one side has stopped pulling, giving us a rare chance to learn two things: whether all the effort exerted by mrs. clinton is moving the flag, and whether mr. trump<u+2019>s method is a good substitute for a conventional ad campaign. seth j. hill, a political scientist at the university of california, san diego, suggested an approach to get some answers: compare the state of the race in the places where there was little imbalance in advertising with the state of the race in places where mrs. clinton dominated the airwaves and the very few places where mr. trump did, like maine and wisconsin. this would provide a glimpse of the effects of the ads. i found that mrs. clinton<u+2019>s vote margin decreased, on average, by a third of a point in places where mr. trump out-advertised her. but her margin increased, on average, by 2.6 points in places where she out-advertised him. in the forgotten states where neither candidate is advertising very much <u+2014> those places with fewer than 100 total ads <u+2014> mrs. clinton<u+2019>s margin is up four points since june. maybe you<u+2019>re thinking the candidates are just good at guessing where they are most likely to pick up votes, and they put their ads in those places. that<u+2019>s a reasonable concern. to try to get a sense of whether the effect of the ads is actually the effect of selecting the places to advertise (based on where the race is most likely heading), i looked at the same relationship using president obama<u+2019>s share of the vote from 2012 in each state and found the opposite relationship. in places where mr. trump dominates the airwaves, mr. obama won 60 percent of the vote in 2012. in places where mrs. clinton is winning the ad war, the president won only 50 percent of the vote. the ads in 2016 do not predict the 2012 election result, suggesting that the states where mrs. clinton both advertised and picked up big gains over the summer were not on their way to some preordained outcome, like the one in the last presidential contest. she is out-advertising mr. trump in places where mr. obama<u+2019>s race was quite close, and she is gaining vote share. this suggests an impact from mrs. clinton<u+2019>s campaign and an effect from the imbalance between the two candidates<u+2019> efforts. essentially, the more candidates out-advertise their opponents, the more they pick up in vote share. the effects are small, but they add up. in the battleground states, mrs. clinton<u+2019>s share of ad volume was quite high over the summer <u+2014> in the 80 percent range <u+2014> suggesting that gains of about 2.5 points in places like ohio and florida were mainly because of her effort. in arizona, where she nearly had the airwaves to herself and where she is continuing to make substantial ad buys, the imbalance most likely delivered her an additional 3.8 points of vote share. in case you are wondering about my method, since we don<u+2019>t get state polls (especially in non-battleground states) on a regular basis, i needed a way to measure the race in each state. to get at this, i used the upshot<u+2019>s daily forecast of mrs. clinton<u+2019>s vote margin in each state. details on how the forecasts are generated can be found here . daily polls in every state would be better, but the forecast margin is a relatively good proxy. if anything, it underestimates the effects of campaigning, since by design it is looking for similarities across states and over days in order to render estimates between the dates of actual polls. to measure the ad imbalance starting in june and going to the second week of october, i used data from kantar media/cmag to calculate mrs. clinton<u+2019>s share of presidential campaign advertising in each state. a few media markets cross state lines; where this happens i attributed the ads to the state in which the majority of the market resides. there were no states that avoided advertising entirely during this period, but there were some with an extremely low total volume of ads. twenty-six states had fewer than 100 ad airings, while nine had more than 5,000. voters in florida and ohio have had the chance to see over 120,000 television ads combined <u+2014> most of them for mrs. clinton. since i couldn<u+2019>t isolate the effects of ads from the effects of all the other things candidates do to win votes, like knocks on doors and phone calls, the effects are best thought of as reflective of variations in campaign effort as represented by the imbalance in ads. all told, from june to october there were five states with more than 100 ads in which mr. trump dominated, and 18 states with more than 100 ads in which mrs. clinton dominated. the largest imbalance for mr. trump was in michigan, where he out-advertised her by 1,567 ads. the largest clinton advantage was in florida, where she is up by nearly 44,000 ads. over this same period, the upshot<u+2019>s state models estimated that mrs. clinton<u+2019>s vote margin went up by an average of about a half percentage point. in battleground states where the race is traditionally hard fought, places like north carolina, ohio, pennsylvania and florida, her margins have gone up since june <u+2014> and sometimes by quite a bit. in pennsylvania, the increase was nearly six points. in florida and nevada, it was about two points. the question is whether the changes in her margins had anything to do with her ad campaign <u+2014> and mr. trump<u+2019>s <u+2014> or were likely to happen anyway. to get a sense of the effects of mrs. clinton<u+2019>s ad advantage, i plotted the change in her share of the total ads from june until october in each state with more than 100 ads against the change in her vote margin in that state as calculated by the upshot<u+2019>s state models. the relationship is positive. all this suggests that mr. trump<u+2019>s strategy, while efficient in terms of costs, may not be effective in terms of persuasion. he has let mrs. clinton dominate the ad war in competitive battleground states and it seems to be costing him votes. lynn vavreck, a professor of political science at u.c.l.a., is a co-author of <u+201c> the gamble ,<u+201d> about the 2012 presidential campaign. follow her on twitter at @vavreck . the upshot provides news, analysis and graphics about politics, policy and everyday life. follow us on facebook and twitter . sign up for our newsletter. a version of this article appears in print on october 20, 2016, on page a17 of the new york edition with the headline: do tv campaign ads matter? trump gives rare chance to find out. order reprints | today's paper | subscribe
do campaign ads matter? donald trump offers a rare chance to find out
lynn vavreck