With anticipation building for a new round of U.S. tariffs against China, UBS AG warned the resilience in American stocks is under threat.

They may suffer their biggest retreat since April if President Donald Trump imposes tariffs at the high end of an expected range, according to UBS strategist Keith Parker. The S&P 500 could drop 5 percent if the U.S. levies 25 percent tariffs on an extra $200 billion of Chinese goods, a scenario that he said has not been priced into the market. The firm’s economists expect 10 percent tariffs to be enacted by the end of September.

The public-comment period on the additional tariffs is scheduled to conclude Thursday. Trump plans to impose them once the deadline passes, Bloomberg reported last week, citing people familiar with the matter. The tariff rate could range from 10 percent to 25 percent.

U.S. stocks have shrugged off trade tensions over the past few months, in stark contrast with their Chinese counterparts, as two-way trade talks show little signs of progress. The S&P 500 rose five straight months through August, notching record highs, while the Shanghai Composite Index has slumped into a bear market.

A worse-than-expected tariff announcement could trigger a selloff at a time when companies enter a buyback blackout period, removing a key support for the market, Keith said. Earlier Thursday, China said it will take necessary retaliatory measures based on U.S. moves.

“A 25 percent tariff rate could be construed as an escalation and would have a greater earnings impact,” Parker wrote in a note to clients. “The corporate bid will slow again in September and reach a trough in early October, which may coincide with the implementation of tariffs.”

Parker is the latest Wall Street strategists weighing in on looming risks for stocks. Earlier this week, Goldman Sachs and Citigroup warned investors over potential dangers, citing everything from euphoric sentiment to elevated valuations to a maturing economic cycle.

To Parker, any pullback is likely to be brief. October will probably bring some relief to the market as earning season starts and midterm elections approach. A UBS study of past election cycles found that stocks tend to start to rally in early October. The pattern may repeat as the threat of losing Congress seats prompts “more market friendly rhetoric,” according to Parker.

Another force to counter the trade risk could come from the Federal Reserve, which is expected to raise interest rates this month for an eighth time since 2015. Should the central bank refrain from further hikes this year, that would help the market, the firm says.

“The Fed skipping a hike in December, our economists view, could provide an important offset for trade risks, particularly since the USD has been a key driver of relative equity returns,” Parker wrote.