Daniel W. Drezner is a professor of international politics at the Fletcher School of Law and Diplomacy at Tufts University and a regular contributor to PostEverything.

President Obama is arriving in the U.K. to a storm of criticism from those campaigning for Britain to leave the E.U. (Reuters)

From the earliest moments of Barack Obama’s candidacy for president, there was an interesting debate about whether he would enhance America’s soft power in the world or not. Over the past eight years there have certainly been some data points that offer evidence of Obama’s soft power — but there has also been a legitimate debate about whether it means all that much for American foreign policy.

I bring this up because I think we’re about to witness an interesting test of whether Obama’s soft power still matters in the United Kingdom. Obama’s recent and decent poll numbers in the United States obscure the fact that he’s even more popular in the rest of the world. According to Pew, last year 76 percent of Britons expressed confidence in Obama doing the right thing in foreign affairs.

This matters because the U.K. is preparing for a referendum two months from now on whether to exit the European Union, dubbed Brexit. Concerned Americans have begun to weigh in on the issue. On the one hand, you have eight former U.S. treasury secretaries writing a letter to the London Times saying that Brexit would be a reeaaaaallllly bad idea. On the other hand, you have former U.S. ambassador to the United Nations John Bolton being in favor of Brexit.

The last time something like this came up in the United Kingdom was the Scottish referendum for independence 18 months ago. Obama kept mum about that until the last minute, when he tweeted out his opposition. That seemed to turn out well for American interests.

That intervention, however, was oblique compared to today’s Daily Telegraph op-ed by the president, in which he was pretty explicit about Brexit:

As citizens of the United Kingdom take stock of their relationship with the EU, you should be proud that the EU has helped spread British values and practices — democracy, the rule of law, open markets — across the continent and to its periphery. The European Union doesn’t moderate British influence — it magnifies it. A strong Europe is not a threat to Britain’s global leadership; it enhances Britain’s global leadership. The United States sees how your powerful voice in Europe ensures that Europe takes a strong stance in the world, and keeps the EU open, outward looking, and closely linked to its allies on the other side of the Atlantic. So the US and the world need your outsized influence to continue — including within Europe.

It would be safe to say that this has prompted something of a reaction. Pro-Brexit leaders have respond by… um … er … however one would like to characterize London Mayor Boris Johnson’s response to Obama. Let’s just say that they’re not thrilled.

The Remain campaign, however, seems much happier, as Time’s Tara John reports:

“He is a rockstar,” Labour Party MP Stephen Kinnock told TIME. “Brexiters are having a hissy fit because they know what President Obama has to say is highly influential and the British people have a natural affinity with our cousins from across the pond.”

Citing a recent poll by the Guardian that found turnout by young Brits would be key in keeping the country in the E.U., Kinnock believes Obama’s popularity in the U.K. will be enough to “galvanize” younger people in the June 23 referendum.

Indeed, even before Obama’s op-ed dropped, the Financial Times opined that, “the pre-emptive anger of some in the Leave campaign, accusing Mr Obama of hypocrisy and questioning his affection for the UK, only underlines how effective an intervention from him is likely to be.”

The truly interesting question will be the effect on U.K. public opinion polls. There were already signs that, as the debate shifted to the economic realities of exiting the European Union, the British public was shifting against the idea of Brexit. This highlights the fact that there are a lot of other forces at play here beyond Barack Obama.

Still, if that public shift against Brexit halts or reverses itself, one could argue that Obama’s soft power has been tapped out. If not, however, then the president still has some soft power left as he closes out the last year of his presidency.