"We risk the possibility of driving even more jobs overseas," he said.

Pfizer, based in New York, has bid more than $100 billion to acquire London-based AstraZeneca Plc. Pfizer plans to largely keep its operating headquarters in the U.S. even as it moves its legal address to the U.K. for the lower tax rate.

Pfizer shares declined immediately after Wyden's comments and were trading at $29.16 in New York at 3:38 p.m. after reaching an earlier intra-day high of $29.44. Pfizer officials had no comment today.

Other U.S. companies have lowered their tax bills by shifting legal addresses to low-tax jurisdictions including Switzerland, Ireland and Bermuda.

Passing any short-term curbs on such shifts will be difficult, because Congress has been deadlocked for years over international tax policy. Republicans say the trend of companies obtaining non-U.S. addresses through mergers shows the need for a more complete revision of the tax code, which isn't likely until 2015 at the soonest.

Levin, a Michigan Democrat, didn't say how his bill would limit such transactions. Democrats have supported earlier proposals to tax companies at domestic rates if they are managed and controlled in the U.S. even if their headquarters are overseas.

Shifting 'Burden'

Companies that make such moves "benefit from the protections and services the federal government provides, including patent protection, research and development tax credits, national security and more," Levin said in an e-mailed statement. "They shouldn't be allowed to shift their tax burden onto others."

The lawmakers' efforts aren't a surprise and aren't likely to be successful, Princeton University health economist Uwe Reinhardt said in a phone interview.

"These inversions, they are like a 2×4 into the face of Congress, saying pay attention to this," Reinhardt said. "How many people do they have on their side? It would have to go through the House. What's the chance?"

"It's there, it's noise, but I'm not sure Congress is going to address it," he said. "They should really address the more fundamental thing, which is how we tax corporations."

Pfizer would join at least 19 other companies making or contemplating similar transactions since 2012, including Chiquita Brands International Inc. and Omnicom Group Inc., the largest U.S. advertising firm. A stalled proposal from President Barack Obama to limit such deals would raise $17 billion over a decade.

In announcing the offer for AstraZeneca on April 28, Pfizer Chief Executive Officer Ian Read said he didn't think the company's interest in a lower tax rate and maximum return to shareholders would be "a conflict with the interest of the U.S. government."

The spate of inversion deals mirrors what happened in 2001 and 2002, when companies including Ingersoll-Rand Plc and Cooper Industries Plc moved abroad.

Then, Congress effectively imposed a moratorium as the top members of the Senate Finance Committee announced plans to advance legislation and said any bill would be retroactive to that date.

Without saying what he would do, Wyden said retroactivity was one possibility.

Senator Orrin Hatch of Utah, the top Republican on the Finance Committee, said inversions were a "big problem" and that he was looking at the issue carefully and talking with Wyden.

"So far," Hatch said, "I have not agreed with him."

Shaheen Proposal

Senator Jeanne Shaheen, a New Hampshire Democrat, has introduced a measure that would require companies based abroad to be treated as domestic if they are managed and controlled in the U.S. The proposal, which would raise $6.6 billion, hasn't been scheduled for a vote.

Pfizer's offer has also attracted scrutiny from U.K. lawmakers concerned about job losses. The U.S. company told Prime Minister David Cameron last week that it would keep at least 20 percent of the combined company's research and development workforce in the U.S. for at least five years and retain "substantial" manufacturing facilities at AstraZeneca's site south of Manchester. Keeping research operations in Britain enables Pfizer to say that products were invented in the U.K., and allows the company to pay the country's lower tax rate, said Fabian Wenner, an analyst at Kepler Chevreux in Zurich.

Pfizer's Read is set to testify at two separate British parliamentary committees next week. He is scheduled to appear before the Business, Innovation and Skills Committee on May 13 and the Science and Technology Committee on May 14.

Bloomberg News

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