REUTERS/Kevin Kolczynski
Donald Trump.
"The fact that Pfizer is leaving our country with a tremendous loss of jobs is disgusting," he said in a statement to Business Insider.
The Republican presidential front-runner added, "Our politicians should be ashamed."
The $160 billion merger would allow Pfizer to relocate its headquarters from the US to Ireland, where Allergan is incorporated and the corporate tax rate is much lower.
This is an example of a so-called corporate inversion, a hot-button issue on the 2016 campaign trail. Trump has been particularly critical of US companies that shift their operations oversees. For example, he has repeatedly vowed to never eat Oreo cookies again to protest the Nabisco moving a plant to Mexico.
The two leading Democratic contenders - Sen. Bernie Sanders (I-Vermont) and former Secretary of State Hillary Clinton - also blasted the Monday mega-merger announcement.
"For too long, powerful corporations have exploited loopholes that allow them to hide earnings abroad to lower their taxes. Now Pfizer is trying to reduce its tax bill even further," Clinton said in a statement. "This proposed merger, and so-called 'inversions' by other companies, will leave US taxpayers holding the bag."
"The Pfizer-Allergan merger would be a disaster for American consumers who already pay the highest prices in the world for prescription drugs," Sanders added. "It also would allow another major American corporation to hide its profits overseas."
For its part, Pfizer defended the merger as "a great deal for America," and not just about the tax benefits it would receive by relocating to Ireland.
"It allows us to continue to sustain an investment of approximately $9 billion mainly spent in the United States," Pfizer chief executive Ian Read said in an interview with Allergan CEO Brent Saunders and CNBC's Meg Tirrell, following the Monday announcement.
"We have 40,000 combined employees in the United States," he added. "So I think it's a great deal."