(CNN) If Democrats want to win in 2020, they have to sharpen their attacks on the Donald J. Trump economy.

"If the economy a year from now is the same as it is today, or roughly so, then the power of incumbency is strong and Trump's election odds are very good, particularly if Democrats aren't enthusiastic and don't get out to vote," the report's authors write.

Its model has correctly predicted every presidential result since 1980, except for 2016, when its forecast missed the mark on the Electoral College outcome. The economists have since tweaked their methodology, and will use three election models instead of one.

At the last debate, you did not hear the Democrats on the stage demolishing Trump's branding of the economy. Sure, they tackled regulating and even breaking up big tech companies. But no new ground was forged there.

Senator Elizabeth Warren, sharing new front-runner status, was hammered (and she dodged) about whether the middle class would pay higher taxes for Medicare for All. Senator Bernie Sanders said they would, but would make up for it with lower health care costs.

Asked about the threat of automation obliterating jobs over the next decade, Warren blamed bad trade deals, while tech entrepreneur Andrew Yang touted his "freedom dividend," implementing a Universal Basic Income of $1,000 a month, and Sanders pushed an infrastructure plan that he says would employ 15 million people.

Sanders and billionaire Tom Steyer hit on income inequality and taxing "the billionaire class." And Steyer said he would "undo every Republican tax cut for rich people and major corporations."

Sen Amy Klobuchar vowed to repeal tax reform and slammed the president's character. "...Donald Trump, the guy that after that tax bill passed went to Mar-A-Lago, got together with his cronies and said 'Guess what? You guys all got a lot richer,'" she said. "That was the one time in his presidency he told the truth."

It's a potentially powerful line of attack for Democrats. The president's corporate tax cut just didn't have the sizzle the president promised. Signed into law December 2017, GDP has struggled to stay above 3 percent for more than a quarter. Track it here

Companies used their newfound largesse to buy back shares of their stock. That benefits shareholders, not necessarily the public. It was not the rocket-fuel to GDP that the president and his team promised.

In a lost opportunity, the Democrats didn't address the president's trade war with China and what they would do to either end it or keep the pressure on the Chinese. (In fact, the only China attack was Sanders against former Vice President Joe Biden's record on free-trade agreements.) Nor did they question the president's hyperbole and exaggeration on how good American economy is.

Just yesterday the International Monetary Fund forecast weaker global growth next year and predicts US that growth will continue to slow. Rising trade and geopolitical tensions has put economies in a "precarious" position. We're talking the slowest global growth since the financial crisis.

The US — faring better than most — has a target growth of 2.1 percent for 2020, the IMF said.

But folks, this is hardly the 4 percent growth the president promised from his tax cuts and deregulation. (At his most hyperbolic he promised 6 percent GDP.) This is slower than the even 2.4 percent the IMF still expects for 2019.

What's wrong? Trump's trade war, for one. Businesses spend less and invest less when they don't have a clear picture of the future. The presidenet's trade "deal" last week with the Chinese is nothing more than an agreement to prevent mutually-assured destruction. Tariffs are still in place, with another tariff deadline looming in December.

And we have no idea if the Chinese will live up to their promises on IP and currency. (In fact, the agreement is not even in writing yet.) A plus for the president politically, is that the Chinese are buying US agricultural products, but that isn't necessarily news, given that purchases began in August.

At the debate, the Democrats focused their fire on each other, or more broadly on the president's fitness for office and the impeachment inquiry. They failed to point out the irony of arguing over how expensive Medicare for All would be, when Trump Administration tax cuts and huge government spending are ballooning the budget deficit and piling up the national debt.

The president has proven masterful at controlling the message, using his microphone and Twitter account to exaggerate success. Just 24 hours ago, the president tweeted about median household income figures. "Just out" he declared. "Median household income it's at the highest point ever...in the history of our country." "Tough numbers for the Radical Left."

Facts are not so important to his narrative. For example, that report came out a month ago and household income growth was statistically unchanged from last year, which by the government's own analysis was just above $64,000.

Obviously with a Democratic field so big--12 candidates on stage!--each candidate is trying to break free from the rest. And that leaves the president to continue branding the economy on his own terms.