The Thursday statement outlined broad principles for the Republican tax reform effort: simplifying the tax code, lowering rates for corporations and small businesses, and cutting the burden on middle-class Americans. But it did not offer new details on possible corporate or income tax rates, revenue neutrality or other specifics that could be included in the joint tax plan.

The officials added that they expect a bill to start moving through relevant committees "this fall," reiterating an earlier timeline.

"While we have debated the pro-growth benefits of border adjustability, we appreciate that there are many unknowns associated with it and have decided to set this policy aside in order to advance tax reform," House, Senate and White House leaders working on a tax plan said in a joint statement Thursday.

Top Republicans driving the GOP tax reform effort have given up on border adjustment.

U.S. House Speaker Paul Ryan speaks at his weekly press conference on Capitol Hill in Washington, DC, on July 27, 2017.

The officials who issued the statement — House Speaker Paul Ryan, Senate Majority Leader Mitch McConnell, Treasury Secretary Steven Mnuchin, National Economic Council Director Gary Cohn, House Ways and Means Committee Chairman Kevin Brady and Senate Finance Committee Chairman Orrin Hatch — have recently been meeting to strike a joint plan that the GOP wants to push through Congress this year.

When President Donald Trump won the White House and Republicans held on to both chambers of Congress in November, tax reform became a top priority for the united government, as the GOP sees it as a lever for spurring faster economic growth. The issue has so far simmered in the background as the GOP struggles to reach a deal to repeal Obamacare, another key campaign plank that has repeatedly stalled amid party divisions.

The border adjustment proposal was a key revenue-raising plank of the plan House Republicans unveiled last year. It taxes imports but lets exports go untaxed.

Retailers that get many of their inputs from overseas raised concerns about the proposal and said it would pass costs on to consumers. Numerous senators also knocked the provision, leading to doubts in recent months that it could become part of a joint tax proposal.

Many questions remain about what shape the plan will take. While the White House released a brief summary of its goals for tax reform earlier this year, it has released little about specific policy since.

In a statement responding to the GOP leaders, Finance Committee ranking member Sen. Ron Wyden, D-Ore., said, "Republicans are dripping tax ideas out like a leaky faucet with no specifics to back them up."

In its earlier outline, the Trump administration called for reducing income tax brackets from seven to three, with a top rate of 35 percent and lower rates of 25 percent and 10 percent. The proposal would chop the corporate tax rate to 15 percent from 35 percent.

The White House said there will be a "one-time tax" on the trillions of dollars held by corporations overseas.

It would include various other provisions like the elimination of most deductions.

Questions have grown about how low Republicans can take business or individual rates while keeping the plan revenue neutral.