NEW YORK -- Pfizer Inc. on Monday said it would buy Botox maker Allergan Plc in a deal worth $160 billion to slash its U.S. tax bill, rekindling a fierce political debate over the financial maneuver.

The acquisition, which would shift Pfizer's headquarters to Ireland, would be the biggest-ever tax inversion. The news prompted Democratic presidential front-runner Hillary Clinton to promise to propose measures to prevent the increasingly popular and controversial practice aimed at helping U.S. companies lower their taxes by re-incorporating overseas.

Shares of Allergan and Pfizer fell more than 2 percent as investors learned the merger, which would create the world's largest drugmaker, would bring lower cost savings than they had hoped. It also would delay a decision by Pfizer on whether it would sell off its division consisting of products facing generic competition.

President Barack Obama has called inversions unpatriotic and has tried to crack down on the practice.

Senator Bernie Sanders, another Democratic candidate for president, called on the Obama administration to stop the deal, which "would allow another major American corporation to hide its profits overseas."

"Congress also must pass real tax reform that demands that profitable corporations pay their fair share of taxes," Sanders said.

To avoid potential restrictions, the transaction was structured as smaller, Dublin-based Allergan buying Pfizer, although the combined company will be known as Pfizer Plc and continue to be led by Chief Executive Officer Ian Read.

The U.S. Treasury, concerned about losing billions in tax revenue, has been taking steps to limit the benefits of tax inversion deals, but it admitted last week that it would take legislation from Congress to stop such moves.

The Allergan acquisition will delay New York-based Pfizer's decision on whether to sell off its lower-margin unit by two years, until late 2018, the company said.

The deal enhances offerings from both Pfizer's faster-growing branded products business, with additions like Botox, and its older established products unit. Still, investors had been hoping Pfizer would sell off the lower-margin business in 2017, a move now put off by the time required to integrate Allergan.

"The only thing I'd really say I'm disappointed about is Pfizer's postponing their break up," said Gabelli Funds portfolio manager Jeff Jonas. He called the delay decision "pretty conservative and a little late."

Others were disappointed by other aspects of the deal, including the projected cost savings, and a lack of details on potentially increased share buybacks.

"Synergies of $2 billion plus in the third year are less than the $4 billion we had estimated in year 1," said Cowen and Co analyst Steve Scala.

On a conference call with analysts, Pfizer said the merger would give it enhanced access to its tens of billions of dollars parked overseas and allow for more share buybacks, dividend payments and business development. The combined company would have annual sales of about $64 billion.

It was not immediately clear how many jobs would be lost as a result of the deal, which is expected to close in the second half of 2016.

Allergan CEO Brent Saunders will become president and chief operating officer of the combined company, with oversight of all commercial businesses.

Read, who has long sought to slash Pfizer's U.S. tax rate, said the deal would help put the company on "on a more competitive footing" with overseas-based rivals.

The company had estimated it would pay about 25 percent in corporate taxes this year, compared with about 15 percent for Allergan. Pfizer Chief Financial Officer Frank D'Amelio said he expected a combined tax rate of 17 percent to 18 percent by 2017.

The deal comes some 18 months after the failure of Read's initial attempt at an inversion, a $118 billion bid to acquire Britain-based AstraZeneca Plc that ran into stiff opposition from that company's management and UK politicians.

Saunders said the combination would provide access to about 70 additional worldwide markets for Allergan products, such as Botox wrinkle treatment, Alzheimer's drug Namenda and dry-eye medication Restasis.

For 166-year-old Pfizer, Allergan would be the fourth huge acquisition over the last 15 years - one for each of the last 4 CEOs - following purchases of Warner-Lambert, Pharmacia and Wyeth.

This also caps a record year for healthcare mergers and acquisitions, taking their cumulative value in 2015 to more than $600 billion..

They include prior big deals involving Saunders, such as the $70.5 billion acquisition of Allergan by Actavis, which then took the Allergan name, and an agreement to sell that company's huge portfolio of generic drugs to Teva Pharmaceutical Industries for $40.5 billion.

Allergan and Pfizer estimated their merger would increase earnings per share by 10 percent, excluding special items, in 2019 and add by a high-teens percentage rate in 2020.

The deal values Allergan shares at $363.63 each, about 16 percent more than their closing price of $312.46 on Friday. Pfizer shareholders would control of 56 percent of the combined company. The record-breaking deal includes $8 billion in debt, Pfizer said.

Allergan shareholders would receive 11.3 shares in the combined entity for each of their shares.

Pfizer stockholders can get cash or one share of the combined company for each of their shares, but the aggregate amount of cash must range from $6 billion to $12 billion.

Plans call for four current directors of Allergan, including Saunders and Executive Chairman Paul Bisaro, to join Pfizer's 11-member board, the companies said.

Reports that the companies were in talks emerged a month ago.

Pfizer was advised by Guggenheim Securities, Goldman Sachs & Co, Centerview Partners and Moelis & Co. Its legal advisers are Wachtell, Lipton, Rosen & Katz; Skadden, Arps, Slate, Meagher & Flom LLP and A & L Goodbody.

Allergan was advised by J.P. Morgan, Morgan Stanley and Cleary Gottlieb Steen & Hamilton LLP. Latham & Watkins LLP and Arthur Cox are its legal advisers.