Trucks travel on an overpass to the World Trade Bridge in Laredo, Tex. (AP)

With the next round of North American Free Trade renegotiations underway, Canadian Prime Minister Justin Trudeau arrives in D.C. today to lobby hard on all the NAFTA benefits — targeting vocal critic President Trump but also the broader American public.

Trump speaks loudly and frequently about trade matters — and is negative on trade. He’s not alone in this respect, as this is classic campaign-trail rhetoric for most U.S. politicians. As a result, defenders of trade agreements face a skeptical public.

Candidates tend to talk tough on trade

During the September 2016 presidential debate, candidate Trump called the two-decade-old NAFTA “the worst trade deal maybe ever signed anywhere.” And Trump repeatedly denounced current agreements for failing “to stop these countries from stealing our companies and our jobs.”

Democratic candidate Bernie Sanders voiced similar sentiments during the debates and in stump speeches across America, condemning “disastrous” trade agreements.

While Trump and Sanders expressed similar views on trade, so do many other elected officials — even those who actually vote for trade agreements. When politicians speak about trade, they seldom do so in a positive light.

… even if the candidate actually voted for trade deals

For the 2000, 2002, 2004 and 2008 U.S. presidential and midterm elections, the Wisconsin Advertising Project has compiled and indexed campaign advertisements from the country’s largest media markets. I looked at the 385 identified as “trade-related,” then further coded the tone of each ad.

Here’s what I found: When candidates discuss trade, the overall tone is negative. In the 2000 election cycle — just six years after completion of the NAFTA negotiations — only three candidates (none of whom were presidential candidates) ran a trade-related campaign ad. Only one of those, in support of Sen. John Ashcroft (R-Mo.), offered a pro-trade message.

In the 2004 election cycle, discussion of trade increased but was again one sided. Of the 115 trade-related advertisements, 113 promoted the need to protect the U.S. market from imports and increased outsourcing of jobs abroad. The two remaining ads were not pro-protection, but only one was explicitly in favor of imports.

In the 2008 election cycle, 95 percent of the airtime devoted to trade offered a negative spin — 130 of the 184 trade-related ads explicitly supported the need for protection, even though the majority of the incumbents that year had supported trade liberalization during the previous legislative session.

The 2012 election was no different. Of the 143 trade-related ads, only three were pro-trade. Again, many of the negative trade-related ads were at odds with congressional voting records.

In 2012, Public Citizen called out 18 Democrats and Republicans who supported the Korea Free Trade Agreement, yet still ran ads against offshoring. Public Citizen highlighted that six Republican incumbents were running ads against trade liberalization despite a “100 percent track record of support for every single NAFTA-style trade deal arising under their tenure.”

Would pro-trade political campaign ads influence beliefs about trade?

If there were more positive campaign ads about trade, would it make a difference in public opinion? It’s difficult to answer this question because politicians are strategic about when and where they run trade-related campaign ads — and positive ads are few and far between.

For my book “American Opinion on Trade: Preferences without Politics,” I fielded an online survey experiment using a pro-trade ad that was never nationally broadcast for Sen. John McCain’s (R-Ariz.) 2008 presidential campaign. McCain looks directly at the camera and declares his support for the Colombia Free Trade Agreement.

To fuel our economy, we must create more jobs for Americans and for our neighbors to the south. With better jobs, more of them will be able to stay in their country. We can’t go back on our word on free trade promises with Mexico, Canada, Central America, or anyone else. We must encourage more trade agreements to create more jobs on both sides of the border. That’s why I’m behind the Colombian Free Trade Agreement. I’m John McCain, and I approve this message.

The 900 individuals in my survey pool were randomly sorted into three groups: (1) those asked to watch the 30-second trade-focused McCain advertisement “Colombia Free Trade” (the positive treatment); (2) those asked to watch a 30-second employment-focused McCain advertisement “Jobs for America” (the placebo treatment); and (3) those who did not view a video (the control group). The placebo served as a check that the influence of the “Colombia Free Trade” ad is indeed attributable to the discussion of trade rather than the other aspects of the advertisement specific to McCain himself, the discussion of the economy, or viewing a political ad in general.

I then asked each group a series of questions, including a set on the employment benefits of trade at the national, regional and individual level.

As expected, the group that watched the pro-trade McCain ad was significantly more likely to respond that free trade benefits national employment. The effect was not consistent across all groups — for example, the message more strongly influenced men than women — but on average the pro-trade video increased those expressing positive beliefs about the national benefits of free trade agreements by 3 percentage points and lowered negative beliefs by 4 percentage points.

Why don’t pro-trade politicians run pro-trade ads?

A pro-trade message, then, may shift some voters’ views on trade, but running such ads incurs high costs. Other research shows that while foreign policy experts see the benefits of trade at the national level, Americans are far more negative, and don’t buy into this argument. Individual ads may nudge the direction of the beliefs, but they also put a spotlight on a thorny issue — one in which many politicians differ from their constituent base.

This gap showed up even within my own experiment. I found that while beliefs were more positive among the group that viewed the pro-trade ad, respondents also were more likely to express an opinion on trade agreements. That opinion supported increased limits on trade (29 percent) almost as much as it opposed increased limits (33 percent).

In such an environment, voter indifference may be politically valuable, especially for incumbent politicians with less political stature than McCain. I found that voters who are indifferent on trade are just as likely to vote for the incumbent as those who know and match a politician’s position on trade. Politicians may tend to focus on issues with larger potential vote swings.

As current U.S. trade policy faces a redesign, it likely will fall to the business world to provide pro-trade messages. And while some industry ads, like this recent attack by the National Retail Federation on the proposed border tax, are highly entertaining or heartfelt, run off-cycle and online, most Americans will not hear them before the next cycle of protectionist campaign ads.

Alexandra Guisinger is an assistant professor of political science at Temple University. Her research focuses on domestic and international reactions to countries’ trade, capital and exchange rate policies. Follow her on Twitter @A_Guisinger.