American companies that merge with smaller foreign firms in an effort to leave the nation's tax system should be charged an "exit tax," Democratic presidential front-runner Hillary Clinton said on Monday.

The former secretary of state's proposal, which The Wall Street Journal said could have discouraged technology company Medtronic from putting its tax address in Ireland and could complicate similar efforts by drug maker Pfizer, is even more restrictive than a similar plan President Barack Obama had put forward earlier this year.

And while the president's suggestion has run into staunch opposition in the Republican-controlled Congress, Clinton insisted that plans such as Pfizer's "leave U.S. taxpayers holding the bag." She promised to elaborate on the details of her proposal at upcoming campaign events in the crucial early-primary states of Iowa and New Hampshire, her campaign told Reuters.

Currently, the federal government taxes companies on their worldwide earnings, but does allow them to claim certain foreign tax credits for profits earned abroad, The Wall Street Journal explained. An exit tax might help stall many firms' efforts to earn money abroad in low-tax countries and leave it there, the newspaper added.

Clinton's initiative is part of a broader, tough approach toward financial regulation, which the former New York senator outlined in a New York Times op-ed published on Monday.

"Republicans may have decided to forget about the financial crisis that caused so much devastation - but I haven't," the Democrat wrote in the newspaper. "The proper role of Wall Street is to help Main Street grow and prosper."

Meanwhile, the presidential front-runner received backing from another popular Democrat, liberal Massachusetts Sen. Elizabeth Warren, who has otherwise remained neutral in the party's primary race, The Associated Press pointed out.

"Secretary Clinton is right to fight back against Republicans trying to sneak Wall Street giveaways into the must-pass government funding bill," Warren, a former Harvard Law School professor and long-time consumer advocate, noted on social media.