The Trump administration's policy of upending trade as the world knew it will kick into high gear as September unfolds. On tap are a proposed $200 billion in new tariffs on Chinese goods, auto and auto parts tariffs, and more North American Free Trade Agreement negotiations with Mexico and Canada.

At stake are higher costs for U.S. companies and consumers and the pace of U.S. economic growth. Some economists predict millions of jobs are at risk. As voters get ready for midterm elections in November, they're "getting more worried about the negative economic impact of tariffs," wrote Deutsche Bank economist Torsten Sløk, in a note to investors last week.

Here's what to look for this month:

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China tariffs -- and countertariffs -- may come this week

The Office of the U.S. Trade Representative proposed 10 percent tariffs July 10 on a list of 6,031 Chinese products. Mr. Trump later escalated the potential pain to a 25 percent levy.

Hearings on those tariffs concluded last week, and the last day to file appeals and comments is Sept. 6. President Trump wants to enact the tariffs as soon as possible, according to Bloomberg. Some economists, including UBS's Seth Carpenter, predict the duties may be announced the week of Sept. 10 after paperwork is complete, or even sooner.

However, they may not be imposed until later this year, Height Capital's Clayton Allen recently said in a note. Tariffs are typically declared with an effective date in the future.

Mr. Trump's assertion that he may move as soon as this week "does not indicate that the U.S. will impose tariffs immediately," Allen wrote. "Instead, we expect that the closure of the public comment period on the next $200 billion in China tariffs will allow Trump to finalize and implement the tariffs later in the fall."

These new tariffs would be imposed on top of the $50 billion already in effect. China has readied immediate retaliation with $60 billion in tariffs on U.S. imports. Beijing's negotiators left Washington last month without making apparent progress in multiple disputes. And the Trump administration last month announced a $4.7 billion tranche of a $12 billion package of subsidies to help hard-hit American farmers absorb their trade-war losses.

All this may slow the U.S. economy in 2019, Moody's Investors Service said last month. Axios has calculated the dispute will touch about 11 million jobs.

Autos and auto-part tariffs may be finalized this month

Mr. Trump's threat to slap heavy tariffs on imported cars and parts may cost hundreds of thousands of American jobs and raise auto prices in the U.S. by roughly 10 percent, many economists estimate. Echoing their dire predictions are companies, industry groups, global trading partners and even local city governments in U.S. Commerce Department hearings or public statements over the summer.

"The administration seems ready and willing to move forward on auto tariffs," UBS economists led by Seth Carpenter wrote in a recent note. "We expect an announcement on the autos investigation in the first two weeks of September."

In a July study, the Peterson Institute for International Economics estimated the proposed 25 percent tariffs would cause the price of an entry-level compact car to rise between $1,409 and $2,057 in the U.S., while a new compact SUV/crossover, the most popular vehicle in America, would rise by $2,092 to $3,066.

The auto industry is so intertwined among countries that a 100 percent U.S.-made car doesn't exist, the PIIE analysts, led by Mary Lovely, wrote in their report.

Mr. Trump has already used the threat of auto tariffs, which his administration is investigating under a law that invokes a threat to U.S. national security, to push for discussions with Europe. The EU and U.S. suspended potential auto tariffs over the summer, but that truce may not hold for much longer, some observers predicted, based on a Bloomberg interview with Mr. Trump last week.

"The European proposal to remove tariffs on all industrial imports was not enough, which signals the détente with Brussels may be about to end," wrote Gregory Daco, the head economist for Oxford Economics, in a note to clients.

Not everyone agrees on timing, however. Some observers say imposing such tariffs might take longer than the Trump administration implies, in part because of scant political backing.

"Given the lack of political support for auto tariffs in D.C. and within the Republican party, we question Trump's ability to push ahead with aggressive auto tariffs as a result of the section-232 national security tariff investigation into auto imports," Height Capital's Allen wrote in a recent note.

And then there's Mexico, Canada and NAFTA negotiations

The trade understanding between the U.S. and Mexico announced by Mr. Trump last week set into motion a series of events that might lead to an overhaul of NAFTA. Though the treaty has been in force among the U.S., Mexico and Canada since 1994, Mr. Trump said he doesn't want a new agreement to have that moniker. And he's using the potential of a separate agreement with Mexico as leverage to get Canada to go along.

At the least, the Mexico deal would likely boost auto prices for U.S. consumers, PIIE's Chad P. Brown wrote in a recent blog. Still, Congress -- and Mexico and Canada -- must approve the NAFTA changes, according to the 1994 agreement.

U.S. Trade Representative Robert Lighthizer last week sent a note to Congress announcing the administration's intent to sign a final deal in 90 days. That starts a 30-day clock for the White House to resolve all issues -- including Canada's involvement -- and send the final text to Congress.

Adding more urgency: The current Mexican administration under President Enrique Peña Nieto leaves office at the end of November when the newly elected administration takes office.

Negotiations are expected to resume Sept. 5 with representatives from Canada, perhaps the most integrated U.S. trade partner. Wrote UBS's Carpenter: "NAFTA talks will likely continue at a furious pace for the next 30 days."