“They’re just opening us up to the next round of tax shelters on the international side,” he said. “And the I.R.S., underfunded as it is, isn’t going to be able to check anything.”

At the same time, he said, some safeguards aimed at multinationals could still be bypassed. To reduce their home tax bill, companies like Google and Pfizer, for instance, often relocate patents and copyrights in tax havens and then sell use of that intellectual property back to their American subsidiaries at eye-popping prices. These are the higher-than-normal profits — which Senate bill drafters have cunningly called “Gilti” (for global intangible low-tax income) — that Republican bills are trying to stop from leaking out of the tax system.

Multinationals, though, could avoid some of the Gilti tax by shifting more tangible property like production and research facilities abroad.

Other problems arise from the push to reduce the rate on pass-through businesses (sole proprietorships, partnerships and S corporations that currently pay taxes at the individual rate). Lawmakers have advertised the cut as relief for smaller businesses, but high-income investors in hedge and private-equity funds could use the provision to reduce the tax paid on rent and interest income by as much as a third.

Hedge-fund investors have an additional opportunity for a windfall with a simple reporting technique, said Steven M. Rosenthal, a senior fellow at the nonpartisan Tax Policy Center and former legislation counsel with the Joint Committee on Taxation. The funds’ decision to mark their trading positions at their market price (instead of their initial purchase price) would enable any gains to qualify for pass-through treatment at the newly reduced rate of 25 percent instead of being treated as short-term capital gains, at a top rate that nears 40 percent.

The pass-through changes present other tax dodges. Benefits for pass-throughs that provide services — like doctors, lawyers and accountants — are supposed to be phased out for individuals with incomes above $75,000 and for married couples with income above $150,000. But a firm could skirt that limit by creating multiple partnerships with different functions, with one providing services, and the other handling, say, licensing or leasing, said Dan Shaviro, a professor of taxation at New York University Law School who helped write the 1986 tax overhaul.