WASHINGTON—Democratic presidential candidate Bernie Sanders is urging the Treasury Department to impose new tax rules that could halt Pfizer Inc.’s proposed merger with Allergan PLC, or at least make it more expensive.

In a letter sent Friday from his Senate office, Mr. Sanders asked Treasury Secretary Jack Lew to prevent Pfizer and other companies that take foreign tax addresses from borrowing against stockpiled foreign earnings without paying U.S. taxes. He also wants Treasury to limit “earnings stripping,” the intercompany maneuvers companies use after taking a non-U.S. address to load up their U.S. operations with subsidiaries and shift profits to low-tax foreign countries.

“Large multinational corporations should not be able to avoid paying U.S. taxes when children in America go hungry,” wrote Mr. Sanders, a Vermont senator. “Blocking this inversion would not only be sound fiscal policy, it would also act as a strong deterrent to other companies that are contemplating similar tax scams.”

Joan Campion, a Pfizer spokeswoman, declined to comment.

Inversions are transactions in which U.S. companies take foreign addresses, typically by merging with a smaller foreign firm. Stopping them has become a populist cause for Democrats, and both Mr. Sanders and front-runner Hillary Clinton have detailed legislative plans to curb the transactions. Most Republicans, by contrast, favor a broader revamp of the tax system that lowers the corporate tax rate and lightens the tax burden on U.S. companies’ foreign income.