WASHINGTON – Already facing the threat of impeachment for his interactions with Ukraine, President Donald Trump embraced the same tactic Thursday by publicly calling for China to launch an investigation into his leading Democratic rival.

The remarks stunned legal observers, quieted many of the president's allies and provided ammunition to Democrats who are hoping to convince voters that Trump's controversial call in July with Ukrainian President Volodymyr Zelensky wasn't a fluke but rather part of a larger pattern of conduct unbecoming of the Oval Office.

"China should start an investigation into the Bidens," Trump said at the White House minutes after discussing his administration's longstanding trade war with Beijing. "What happened in China is just about as bad as what happened with Ukraine."

Experts pointed to similar open-ended requests Trump or his aides have made to Russia, Ukraine, Australia and, reportedly, the United Kingdom – either to look into the family of Democratic presidential front-runner Joe Biden or to dig up dirt that might discredit former special counsel Robert Mueller's investigation into the 2016 election.

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Democratic lawmakers didn't take long Thursday to begin stitching the pieces together.

"Russia. Ukraine. China. How far will Trump really go to sell out America’s sovereignty to stay in office?" tweeted Sen. Chris Van Hollen, a Maryland Democrat.

"The president of the United States encouraging a foreign nation to interfere and help his campaign by investigating a rival is a fundamental breach of the president's oath of office," said Rep. Adam Schiff, D-Calif., chairman of the House Intelligence Committee and outspoken Trump critic.

House Speaker Nancy Pelosi, who authorized the impeachment inquiry last week, referenced Trump's comments in responding to a request by House Republican leader Kevin McCarthy to suspend the investigation.

In rejecting that suggestion, Pelosi replied to McCarthy that "I received your letter this morning shortly after the world witnessed President Trump on national television asking yet another foreign power to interfere in the upcoming 2020 elections."

Sen. Ron Wyden, D-Ore., said he read Trump’s statement as using the leverage of trade threats for personal gain.

“Linking trade negotiations to Donald Trump’s wide-ranging effort to fabricate foreign accusations against political rivals is a complete abuse of the powers of the president,” Wyden said.

Democrats say they are alarmed by a July 25 phone call between Trump and Zelensky in which the president repeatedly pressed to investigate Biden and his son Hunter, who once had business interests in Ukraine. Trump raised the requests after an extended conversation with Zelensky about U.S. aid to Ukraine the White House had delayed.

Trump describes the call as "perfect" and has denied that the foreign aid was used as leverage to pressure Zelensky.

Like the Ukraine call, Trump's remarks on China came within minutes of him raising U.S. power to punish its foes. Trump has often relied on a carrots-and-sticks approach in his trade policy,but his words Thursday took on new meaning for some critics given the ongoing scandal.

"If they don't do what we want," Trump said of China, "we have tremendous, tremendous power."

Others pointed to Trump's controversial statement in 2016 when he hinted that Russia could "find the 30,000 emails that are missing" from Democrat Hillary Clinton's email server. Trump later explained away the comments as a joke, but Mueller's report noted Russian hackers targeted Clinton’s personal office hours later for the first time.

"If this is another instance in which he is incentivizing, encouraging, calling on another country to perform opposition research to further his own personal political goals, then I do think it’s problematic, and not just for political reasons," said Kent Greenfield, a law professor at Boston College. "It's part of a pattern and practice of the president breaching his oath of office, which requires him to be faithful to the Constitution."

Republican lawmakers were slow to react to the president's latest remarks on China. For the most part, those lawmakers have stood by the White House and have lashed out at Democrats for announcing last week that they would launch an impeachment inquiry.

Trump's call to China came hours after a USA TODAY/Ipsos Poll found growing support for impeachment. Americans by a 45%-38% plurality support a House vote to impeach Trump, the poll found. By a similar margin, 44%-35% of those surveyed say the Senate, which would then be charged with holding a trial of the president, should convict him and remove him from office.

Vice President Mike Pence defended what he described as Trump's "legitimate questions" about Biden's former business dealings in Ukraine and China.

"I think the American people have a right to know if the vice president of the United States or his family profited from his position as vice president during the last administration," Pence told reporters traveling with him Thursday. "That's about looking backwards and understanding what really happened."

Trump and his allies have also reportedly leaned on officials in Australia and the United Kingdom to dig up damaging information about the Mueller investigation into the 2016 race.

David Rothkopf, a foreign policy analyst and critic of Trump, described the president's overtures to foreign governments as unprecedented.

"Our policy has therefore become exactly what the founders feared: a tool to enrich and empower one man, the president," Rothkopf said.

Contributing: Nicholas Wu, Courtney Subramanian