Former US Secretary of State Henry Kissinger supposedly once asked, "Who do I call if I want to call Europe?"

Last December, amid the tumult of an ever-shifting cast of top officials in the White House, the EU's foreign policy chief returned the joking provocation.

"Now the United States have Europe's number, but it's our turn to ask, 'Who do we call if we want to call Washington?" Federica Mogherini said in an interview.

As EU Commission President Jean-Claude Juncker meets US President Donald Trump on Wednesday to talk tariffs after months of escalating tensions over trade, climate policy, and Trump's perceived hostility toward NATO allies and friendliness toward Russian President Vladimir Putin, foreign diplomats say it's often not clear with whom European leaders should be negotiating.

"I'm not optimistic that there is anyone in the administration who can speak rationally to the president in a way that will get him to change him views," Tony Silberfeld, director of the trans-Atlantic relations program at the Bertelsmann Foundation, told DW.

EU diplomats in DC also say it's next to impossible to identify a reliable conduit who is consistently in Trump's favor, even within his own cabinet. They're not sure who influences the unpredictable president — or how. Yet they insist they're willing and able to carry out business as usual on some fronts by working together through departments at different levels.

Who has the president's ear on trade?

According to the European Commission, the discussions between Juncker and Trump will include how to cooperate on "a wide range of priorities, including foreign and security policy, counterterrorism, energy security, and economic growth." It will be a unique opportunity for the president of the EU's executive body to make his case to the only man who truly seems to speak for Trump — himself.

Robert Lighthizer is one of the people European diplomats talk to

Away from the limelight, EU diplomats also consider the relatively low-profile US trade representative, Robert Lighthizer, to be a key point person when it comes to trade. White House economic adviser Larry Kudlow — who has said publicly that he's "not a big fan of tariffs" — is also seen as a somewhat reasonable negotiating partner, compared to Trump's trade adviser Peter Navarro, who more often aligns himself with the president's hardline approach to trade.

Read more: Donald Trump's EU tariffs 'foolish' and 'damaging'

EU diplomats feel the traditional path of pushing a well-reasoned argument through the appropriate channels to reach the US president is no longer their best bet for advocating their positions. Some believe making the case on Twitter is just as successful as arguing before the National Security Council — and that airing grievances so publicly makes it harder to disagree in a constructive way behind the scenes.

One way EU ambassadors tried to show a united front and get their message across was by jointly publishing an open letter in the Washington Post criticizing Trump's tariffs on steel and aluminum. The optimistic subtitle, "The facts speak for themselves," pointed to a list of reaffirmations outlining the EU and US's shared interests in working together, but perhaps also illustrated the EU missing the point when it comes to how to hold Trump's attention.

Europeans struggle to soothe

So what sort of strategy is effective when it comes to finding common ground with Trump? Not the charm offensive, as French President Emmanuel Macron discovered after a spectacular state visit to Washington in April that featured a helicopter ride and a lavish official dinner but no relief from the steel and aluminum tariffs. Neither does tough talk, as German Chancellor Angela Merkel saw a few days later during her own trip to DC trying to accomplish the same. Juncker apparently has consulted both leaders ahead of his visit this week.

European diplomats aren't sure what it takes to make a deal with Trump's administration

Frustrated European diplomats say it's not clear what, exactly, the administration's expectations are. They complain of being unable to identify the White House's chief complaints beyond nebulous statements from Trump.

"Everyone talks about Trump being the transactional president, but I don't know what the Europeans need to give to be treated as equals," Silberfeld told DW.

Read more: In the Trump era, can the concept of free trade survive?

American officials have tried to convince their counterparts to ignore the erratic messaging from the commander-in-chief and focus instead on what policies the United States actually enacts. Yet it's difficult to ignore statements like Trump's recent declaration that the European Union is a "foe" on trade or his taunting tweet the day before meeting Juncker. "I have an idea for them," he tweeted. "Both the US and the EU drop all tariffs, barriers and subsidies! That would finally be called free market and fair trade! Hope they do it, we are ready — but they won't!"

In this extraordinary time for trans-Atlantic relations, Juncker's trip to Washington will be seen as a success if he can convince Trump not to impose additional tariffs. In a sign of just how adversarial Trump's attitude toward the European Union has become, an even bigger victory would be any indication that the historical allies are willing simply to engage in further dialog.