Wisconsin Republican senator Ron Johnson, a vocal critic of Beijing who has vowed to launch investigations into the Biden family’s alleged relationships with Chinese businesses, declared $57m in income in his first 10 years in office in connection to his ownership stake in a company whose growth has closely been linked to China.

Financial disclosures show the senator’s wealth has sharply increased during his years running for and serving in the Senate thanks to his holding in Oshkosh-based Pacur, a plastics maker where Johnson previously served as top executive.

Johnson is seeking re-election for a third term in a tough contest against Democrat Mandela Barnes, who polls show has a slight edge over the Republican incumbent.

During his first run for public office before his 2010 election, Johnson portrayed himself as a successful businessman who knew how to create jobs. An advertisement he used in both his successful 2010 and 2016 campaigns showed the 67 year-old standing in front of a white board, touting his own record as a manufacturer, a fact that he said made him stand out in a sea of lawyers who serve in the Senate.

A close examination of Johnson’s financial disclosures and other public filings to the Securities and Exchange Commission, legal filings and other public records reveal that Johnson’s wealth was boosted by his company’s ties to another company that was owned and managed by his family, which in turn grew its business in China, acquired businesses in China, and reported having a loan worth tens of millions of dollars from the Bank of China.

In one case, the company run by Johnson’s family sued the US government to try to press for softer trade relations with Beijing, a position that Johnson himself adopted in a rare break with Trump administration policies.

Johnson sold his stake in Pacur in 2020, although documents show that an LLC owned by Johnson and his wife, Jane, still receives up to $1m annually through rent and royalties as owners of the building where Pacur operates.

Pacur was co-founded by Johnson and his brother-in-law in the 1970s. It was in effect closely tied to a larger company called Bemis, which was founded and run by Johnson’s father-in-law, Howard Curler.

SEC documents show that, from about 1998 to 2010, Bemis paid tens of millions of dollars to Pacur, which was a supplier to Bemis. Johnson also personally owned Bemis stock, valued at between $1m and $5.2m on financial disclosure forms. The stock was later gifted to the senator’s family foundation, called the Grammie Jean Foundation.

A spokesperson for Johnson said the senator had no beneficial interest in the foundation.

Bemis, records show, had a steady and growing presence in China under the leadership of Jeffrey Curler, Howard Curler’s son and Ron Johnson’s brother-in-law. The company has plants in China and in 2013, records show, appear to have acquired tens of millions of dollars in Chinese debt in connection to a Chinese acquisition. SEC filings show that Bemis also disclosed in 2016 that it had a $50m Bank of China loan.

Bemis was sold to Australia-based Amcor in 2018 in a deal valued at $6.8bn. The Guardian reached out to Amcor for more details about the $50m loan, which appeared in Bemis filings before Amcor acquired the company. A spokesperson for Amcor said Amcor was not involved in the loan and did not have insights into the transaction.

Bemis was also active on issues related to trade during Johnson’s Senate tenure.

In two cases, the group filed suit against the US government’s policies on trade in China, including one suit in 2018. In the legal action brought by Bemis and Rollprint Packaging Products, the plaintiffs sued the United States to contest a finding by the International Trade Commission in support of tariffs on China. The lawsuit did not appear to proceed beyond the complaint. At the same time, Johnson was a vocal critic of US trade policy against China, marking a rare disagreement with Trump.

Johnson has also made speeches in the Senate that criticized financial transactions by Hunter Biden, which he claimed were tied to “Communist China” and meant that US Joe Biden was “probably” compromised on China.

The Biden family’s “vast web of foreign financial entanglements”, Johnson alleged, had serious implications, and Johnson said he and fellow Republican senator Chuck Grassley of Iowa would continue to investigate them.

“Our challenge is the deep state does not give up its secrets easily,” Johnson said in a speech on the Senate floor in March.

A recent investigation by the Washington Post found that CEFC China Energy, a Chinese energy conglomerate, paid nearly $5m to entities controlled by Hunter Biden and his uncle. But it did not find evidence that Joe Biden personally benefitted or knew about the transactions, which occurred after Biden left the vice-presidency and before he announced his presidential run.

Alexa Henning, a spokesperson for Johnson, denied that Johnson had any connections to his own family company’s previous business interactions with Chinese companies.

“He never had any managerial involvement in, or knowledge of the management actions taken by Bemis Company” and had “no connection to China or conflict of interest there”.

“Until your inquiry, he had no knowledge of Bemis’s business holdings in China or any legal action Bemis was involved in. The Bemis company was one of many customers Senator Johnson’s business sold plastics to,” Henning said.

She also defended Johnson’s position on tariffs. “His belief is politicians in both parties have used China’s trade abuses to demagogue against their political opponents without enacting effective solutions.”

Henning also denied that Johnson accrued $57m in income in connection to Pacur from 2009 to 2020, because she claimed some of the funds were designated as “gross receipts” even though they are listed in financial disclosure forms as “income”.

Gross receipts are the amount of business that an organization reports before stripping out expenses. When asked by the Guardian to explain why Johnson had listed the funds as income in his financial disclosures, and what the Gross Receipts referred to, Henning did not respond.

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