The U.S. Capitol stands in Washington in this July photo. Washington policymakers are bracing for a wave of corporations to renounce their U.S. citizenship over the next few months. (Andrew Harrer/Bloomberg)

Washington policymakers are bracing for a wave of corporations to renounce their U.S. citizenship over the next few months, depriving the federal government of billions of dollars in tax revenue and stoking public outrage ahead of the Nov. 4 congressional elections.

So far this year, about a dozen U.S. companies — including such well-known brands as Medtronic medical devices and Chiquita bananas — have merged with foreign firms and shifted their headquarters offshore to avoid U.S. taxes, analysts say.

Dozens of additional deals are in the works, according to administration and congressional officials, and other companies are quietly contemplating the move. Last month, CVS Caremark chief executive Larry Merlo met with Sen. Charles E. Schumer (D-N.Y.) and urged him to act to stop the rash of expatriations. Otherwise, Schumer said that Merlo warned him, CVS “might be forced to do it, too,” to duck a total tax bill expected this year to approach 40 percent.

“There’s a huge number coming,” Schumer said in an interview. “We hear there are going to be several big announcements in August.”

The maneuver, known as tax “inversion,” has been around for decades, but the pace has accelerated in recent years as U.S. firms have expanded overseas and other nations have adopted lower tax rates. At the same time, company executives have grown increasingly frustrated with Washington, where political gridlock has stymied efforts to reduce a 35 percent federal corporate tax rate that is higher than in any other advanced economy.

“What we’re seeing is one more manifestation of why the business tax structure needs to be fixed,” said John Engler, president of the Business Roundtable, an association of chief executives at some of the nation’s largest corporations. “We’re the proverbial frog that’s being boiled in the water, and a few frogs have decided to jump out.”

Last month, President Obama loudly questioned the patriotism of inverted companies, calling them “corporate deserters” who are abandoning their country “just to get out of paying their fair share of taxes. . . . My attitude is, I don’t care if it’s legal. It’s wrong.”

On Tuesday, Democratic Sens. Elizabeth Warren (Mass.), Richard J. Durbin (Ill.) and Jack Reed (R.I.) urged Obama to “use your authority to reduce or eliminate tax breaks associated with inversions.”

Treasury officials confirmed that they are exploring their options for “administrative actions” that could block inversions or “meaningfully reduce” the associated tax benefits. But any move by the Treasury Department would amount to a “partial fix,” the officials said, adding that “legislation is the only way to fully address inversions.”

Treasury Secretary Jack Lew has said that long-stalled corporate tax reform would be the best response but that Congress must consider more targeted measures in the meantime.

Congressional Republicans are also concerned about the issue. But they are reluctant to adopt legislation to punish companies for escaping a tax system that both parties have long agreed is badly broken.

“Companies have an obligation to their shareholders,” said Sen. Orrin G. Hatch (Utah), the senior Republican on the Senate Finance Committee, “and it’s not in their fiduciary interest to pay a 35 percent tax rate.”

Republicans complain that Obama has done little to achieve his goal of lowering the corporate tax rate to 28 percent. Rep. Paul Ryan (Wis.) noted that when then-Senate Finance Committee Chairman Max Baucus (D-Mont.) joined forces with House Ways and Means Committee Chairman Dave Camp (R-Mich.) to build support for a tax-code rewrite, Obama dispatched Baucus to China as its new ambassador.

Thereafter, “we heard crickets from the administration” on tax reform, Ryan said at a recent breakfast hosted by the Christian Science Monitor. “We have had no constructive engagement in actually tackling the issue.”

Given that history, Republicans said, Obama’s current crusade against inversions smacks of a campaign-season ploy to stir up populist anger and paint the GOP as defenders of corporate tax dodgers — a cohort that is “about as popular as child molesters,” one GOP aide glumly noted.

“Inversions are bad . . . but the president’s rhetoric on this is just horrible,” said Rep. Patrick J. Tiberi (R-Ohio), a senior member of the tax-writing Ways and Means Committee. “We are losing revenue, we are losing corporate headquarters and we are losing jobs. We have to do comprehensive tax reform.”

Last week, Congress left town for its summer break without taking action. But Senate Democrats are laying plans to highlight inversions by advancing legislation in September to outlaw the practice or sharply limit its profitability.

Sen. Christopher A. Coons (D-Del.), a Finance Committee member who has seen companies in his state contemplate inversion, said he understands the GOP’s desire to “wait for corporate tax reform. But the very real harm that tax inversions are doing to our revenue, to our communities and to our innovations pipelines demands that we act on a bipartisan basis or risk the loss of thousands of jobs.”

No one has yet measured potential job loss from the practice, said Mindy Herzfeld, contributing editor to the trade journal Tax Notes International. Inversions typically involve relocating on paper only, with company headquarters and executives remaining in the United States.

But the potential costs to the U.S. treasury are enormous. One measure, by the congressional Joint Committee on Taxation (JCT), suggests that the nation stands to lose nearly $20 billion in tax revenue over the next decade. Former JCT director Edward Kleinbard said he thinks the potential loss is much higher.

“My guess is they didn’t fully reflect the sharknado of inversions that is about to happen,” said Kleinbard, a law professor at the University of Southern California.

One closely watched deal: A push to relocate Walgreens, the nation’s largest drugstore chain, to Switzerland, where it could dodge about $4 billion in U.S. taxes over the next five years, according to an analysis by Americans for Tax Fairness, a union-backed nonprofit organization.

On Wednesday, Walgreens announced it will in fact remain headquartered in Deerfield, Ill., where it was founded 113 years ago. Shares of Walgreens fell sharply in pre-market trading on Wednesday.

At CVS, a Walgreens competitor, spokeswoman Carolyn Castel declined to comment on Merlo’s July 16 visit to Schumer’s Capitol Hill office. In an e-mail, Castel said: “Corporate tax reform that includes a significant rate reduction is urgent. The trends we now see in the market underscore the need for a corporate tax structure that allows U.S. companies to be competitive.”

While technically complex, inversions are conceptually simple: A U.S. firm moves its tax home to a nation with lower rates, often by merging with or purchasing a foreign company. The new company is still subject to a 35 percent rate on its U.S. earnings. But profits earned overseas — previously subject to U.S. taxes upon transfer back to the United States — are subject only to the lower foreign rates.

Ten years ago, inverted companies tended to flee to tax havens such as the Cayman Islands. These days, they are more likely to plant their flags in Europe, where many already have operations. Ireland, a popular destination, imposes a tax rate of just 12.5 percent.

There are other benefits to inversion: In many deals, the foreign “parent” makes a big loan to its new U.S. partner, which in turn makes large interest payments that can be deducted from the company’s U.S. tax bill. Those interest payments often are not taxed in the foreign country, a double windfall.

While the Treasury Department explores its options, Obama has called on Congress to make inversions more difficult by requiring that 50 percent control of the company shift overseas, rather than the current 20 percent. Many Republicans, and some Democrats, oppose that idea, saying it risks making the problem worse by forcing actual foreign takeovers.

Schumer has proposed limiting the ability of inverted companies to write off interest payments to their foreign parents. Rep. Sander M. Levin (D-Mich.), who has introduced separate legislation to deny federal contracts to inverted firms, is working on a similar proposal.

That idea has garnered GOP support in the past, and Democrats say they are serious about assembling bipartisan legislation. But Schumer acknowledged that Democrats stand to gain a powerful political weapon if that effort fails because of Republican opposition.

“This is a very popular issue,” Schumer said. “No American — Democratic, Republican, right wing, left wing — likes to see companies that are American go overseas just to escape taxes.”

Danielle A. Douglas contributed to this report.