WASHINGTON — After six months of deliberations and occasional quarreling, the top Republican tax negotiators in Congress and the Trump administration declared on Thursday that they had united behind a set of common principles that would guide them as they rush to complete the first overhaul of the tax code in three decades — and that they would do it by the end of this year.

The five-paragraph joint statement in many respects raised more questions than it answered, providing fewer specifics than the previous plans released by House Republicans and President Trump. The lack of detail highlighted the challenge that Republicans face as they try to make difficult trade-offs on legislation that could reshape the entire economy.

But the long-awaited sign of progress may help generate momentum behind the Republicans’ agenda as their promise to repeal the Affordable Care Act remains unfulfilled and as infighting plagues the White House.

The most notable development was the jettisoning of the proposed border adjustment tax on imports. Speaker Paul D. Ryan, Republican of Wisconsin, had suggested the tax a year ago as a central plank of the plan, arguing that it would accelerate economic growth and protect American manufacturers. But the idea drew the ire of retailers, energy companies and the billionaire Koch brothers, who invested heavily to undermine it. Mr. Trump was cool to it from the beginning, and many Republicans in the House and the Senate began voicing concerns in recent months.