Brexit could spell more trouble for Trump, who supported leaving

Polls show Clinton lead widening over Trump

Donald Trump plans a major trade speech on Tuesday that the campaign is billing as "declaring American economic independence," a clear reference to the U.K.'s decision to leave the European Union. If trends continue, he could be doing it against a backdrop of markets crashing and the pound hitting lows last seen in the 1980s.

So far, global market reaction to the Brexit vote, which Trump strongly supported, is yet another nightmare for a campaign that hasn't had a lot of good news lately.

At first the Brexit vote seemed like it could be a boost to Trump, highlighting the power of nationalist sentiment and a desire for less free trade. But the extended market rout — and some second guessing among the British people themselves — suggest it could be quite bad for him.

To be fair, the market chaos that followed the Brexit vote could abate. But most forecasters are slashing their estimates for U.K. economic growth following the vote. The United States is also expected to take at least a small hit. That leaves Trump's argument that Brexit was good for the U.K. and serves as a model for U.S. policy on trade — or anything else — quite a bit harder to make.

Hillary Clinton's campaign is already hammering away at Trump over Brexit. The former secretary of state seized on the issue on Sunday, saying the U.S. needs leaders "who understand that bombastic comments in turbulent times can actually cause more turbulence, and who put the interests of the American people ahead of their personal business interests."

She did not mention Trump. But Clinton was clearly referring to Trump's comments that the plunging pound was good for business at his Turnberry golf course in Scotland. "No one should be confused about America's commitment to Europe, not an autocrat in the Kremlin, not a presidential candidate on a Scottish golf course," she said.

Clinton is also using Trump's Brexit response in a new television ad, part of a national cable ad buy that has so far gone completely unanswered by a Trump campaign that has essentially no money.

Presumptive Republican presidential nominee Donald Trump delivers a speech as he officially opens his Trump Turnberry hotel and golf resort in Turnberry, Scotland on June 24, 2016.
Oli Scarff | AFP | Getty Images

The Brexit fallout comes amid other dismal news for the Trump campaign. Two major national polls out over the weekend showed the real estate developer well behind with the NBC News/Wall Street Journal survey finding him down 6 points and ABC News/Washington Post showing him behind by a disastrous 12 points, 51 percent to 39 percent. The internals of The Washington Post poll are even more devastating, with two-thirds of Americans saying Trump is both biased against minorities and unqualified to be president. Nearly one-third of Republicans in the poll said they believed Trump was unqualified.

By contrast, at this point in 2012, presumptive GOP nominee Mitt Romney was essentially tied with President Barack Obama. Romney began to sink over the summer as Democrats saturated the airwaves with ads defining him as wealthy and out of touch with Americans' concerns. Trump now faces the same one-sided paid media landscape but starts from a much weaker position than Romney.

Trump needs to get on the air in a major way and have a blockbuster convention in order to start closing the gap with Clinton. But there is no sign of any big ad buys. And at the moment, few major GOP figures want to go to Trump's convention, much less play a significant speaking role.

The precarious nature of Trump's campaign has fueled a revolt among some convention delegates seeking a vote to free them to select a candidate other than Trump. The "dump Trump" movement is not likely to succeed. The rogue delegates have no alternative candidate lined up, and the apparatus of the GOP is firmly aligned with Trump. But the anti-Trumpers could at least get a vote on the floor, which would mean massive media coverage of their efforts to find another nominee. That's not the kind of free media Trump wants just before he drops the "presumptive" and becomes the GOP's official standard bearer.

None of this means Trump is certain to lose. Some polls in swing states, including Florida and Ohio, show the race as much tighter than the national numbers suggest. And the presidential race is really 50 separate state races. There are no bonus points for getting a larger share of the popular vote, just ask Al Gore. And Clinton also has a net negative approval rating while a majority of Americans disapprove of her use of a private email server while at the State Department.

Still, Trump is now heading into the general election in worse shape than any major party candidate in recent history. And unless market reaction to Brexit flips in the coming days, his brand of robust nationalism will be associated with a disastrous vote in the U.K., and the Clinton campaign will have concrete evidence to say, "See, this is what happens when the Trump worldview actually wins."

— Ben White is Politico's chief economic correspondent and a CNBC contributor. He also authors the daily tip sheet Politico Morning Money [politico.com/morningmoney]. Follow him on Twitter @morningmoneyben.