For the first time in four months, US President Joe Biden will meet with his Chinese counterpart Xi Jinping to discuss a wide range of bilateral and international issues.

However, a possible visit to Taiwan by House Speaker Nancy Pelosi looms over the Thursday meeting, with China threatening a harsh response if she visits the self-governing island democracy Beijing claims as its own territory.

China’s warnings about a Pelosi visit were reiterated by spokesperson Zhao Lijian. Pelosi’s office has yet to say when, or even if, she will make the trip, but the timing is especially sensitive given Beijing and Washington’s heightened tensions over trade, human rights, and Taiwan.

While the United States has sent a Cabinet secretary and high-ranking former officials to Taiwan in recent years, Pelosi’s position as the top Democrat in Congress and second in line to the presidency places her in a different category. Standing up to China has been a hallmark of the speaker’s more than three decades in Congress.

While Biden has no authority to prevent Pelosi from visiting, China’s authoritarian Communist government chooses to ignore the United States’ separation of powers, claiming that Congress is beholden to the administration. The fact that both belong to the Democratic Party, in Beijing’s opinion, reinforces the notion that Pelosi is somehow working with Biden’s approval.

Pelosi would be the highest-ranking US elected official to visit Taiwan since Republican Newt Gingrich visited the island as House speaker in 1997. Gingrich and other prominent Republicans, who are normally harsh critics of Pelosi, have expressed support, stating that China has no right to dictate where Americans can travel.

China has not specified what specific actions it would take in response, but experts believe it could launch additional incursions into waters and airspace near Taiwan, or even cross the Taiwan Strait’s center line. Some have speculated that China might even try to prevent her plane from landing, which would cause a major crisis and is widely regarded as unlikely.

According to US officials, if Pelosi visits Taiwan, the military will increase movement of forces and assets in the Indo-Pacific region.

According to Yu Wanli, a professor of international relations at Beijing Language and Culture University, a possible Pelosi visit makes it even more important that Xi and Biden have a meaningful discussion.

Despite opposition from both administrations, Pelosi’s visit “has been hijacked by US domestic politics, with Republicans and other forces exerting pressure not to show weakness to China,” Yu said.

In deference to China, the United States maintains only informal relations and defense ties with Taipei, but it remains the island’s most important source of military and political support. Legally, the United States is obligated to ensure Taiwan’s ability to defend itself and to regard threats to Taiwan as matters of “grave concern.”

China, which has increased its threat to use force to annex Taiwan if necessary in recent years, opposes all US arms sales and contacts with the island’s government.

It conducts military exercises and flights into Taiwan’s air defense identification zone on a regular basis as a warning to supporters of the island’s formal independence and their foreign allies.

The two sides split during a civil war in 1949, and Taiwanese President Tsai Ing-wen has refused to recognize the island as part of China. Taiwanese public opinion is strongly in favor of maintaining the status quo of de facto independence without further antagonizing Beijing.

Along with Taiwan, the leaders are expected to discuss North Korea’s nuclear program, Beijing’s close ties to Russia, Biden’s efforts to resurrect the Iran nuclear deal, and the status of the Trump administration’s review of tough tariffs imposed on China.

Taiwan was a major topic of discussion during Biden and Xi’s most recent phone call in March, just three weeks after Russia launched its invasion of Ukraine. China has refused to criticize Russia’s move, blames the U.S. and NATO for provoking Moscow and has blasted punishing sanctions imposed on Vladimir Putin’s government and political cronies.