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Given the warnings the drug maker Pfizer provides with its products such as Lipitor and Viagra, the company had to know there was a risk of serious political side effects to its $160 billion merger with Dublin-based Allergan, a deal that has upset Democrats.

The announcement of the proposal, which could help the new company escape billions of dollars in United States taxes, drew a cascade of harsh criticism from lawmakers and renewed a push to crack down on these so-called tax inversions. Senator Bernie Sanders, the Democratic presidential candidate from Vermont, even called on the Obama administration to use any authority it had to block the deal.

Democratic critics said what was especially galling about this tax-driven merger was that Pfizer had benefited from years of federal research, tax incentives and patent protection, and that the announcement came just days after the Treasury Department issued new rules to limit such arrangements.

“We cannot continue to allow Pfizer and other corporations to pretend that they are American while reaping the benefits this country has to offer, yet claiming to be another nationality when the tax bill comes,” said Representative Rosa DeLauro, a Connecticut Democrat who has called for banning companies that move their headquarters overseas from getting new federal contracts.

It is unlikely Congress can do much in the short-term to impede the merger, but Pfizer can expect to face a lot of angry questions from lawmakers and perhaps the Obama administration as it seeks to finalize the deal.

Pfizer-Allergan Merger Reignites Tax Reform Discussion The blockbuster pharmaceutical merger has put new pressure on Congress and the White House to overhaul the corporate tax code.