WASHINGTON - Two top Senate Democrats are demanding details of the Trump administration's delay in restoring Ukraine's trade privileges, which occurred last summer as President Trump was also withholding military and security aid from the embattled U.S. ally.

The twin delays came as the White House sought to pressure the Ukrainian government to investigate the son of former vice president Joe Biden.

"It would raise grave concerns both domestically and internationally if U.S. trade policy were used as a bargaining chip to achieve partisan political ends," Sens. Ron Wyden of Oregon and Robert Menendez of New Jersey, the ranking Democrats on the Senate Finance and Foreign Relations committees, wrote in an October 30 letter to U.S. Trade Representative Robert E. Lighthizer.

The letter was prompted by an Oct. 24 Washington Post story, which detailed the internal White House debate this summer over restoring Ukraine's benefits under a longstanding program called the "generalized system of preferences" (GSP.) The administration in December 2017 had partially suspended Ukraine from the program after Kyiv failed to address repeated U.S. complaints about widespread digital piracy and intellectual property violations.

Lighthizer this summer recommended restoring one-third of the suspended privileges, following Ukraine's implementation of legislation that addressed the U.S. complaints. But in late August, the trade chief withdrew the recommendation after then-National Security Advisor John Bolton warned him that Trump was unlikely to approve any step that would benefit the Ukrainian government, The Post reported.

Bolton's intervention came as the president was telling White House aides that Ukraine would get no U.S. aid until Zelensky publicly stated that his government would investigate Hunter Biden's role as a board member of the Ukrainian gas company Burisma, according to congressional testimony last week by acting U.S. Ambassador William B. Taylor Jr.

House Democrats last month launched an impeachment inquiry into the president's alleged efforts to coerce Zelensky into commencing an investigation of the Ukrainian business activities of Hunter Biden, the former vice president's son.

The Bolton-Lighthizer exchange showed that the administration's suspension of ties to Ukraine may have been more extensive than initially known.

Wyden and Menendez want Lighthizer to disclose whether he discussed U.S.-Ukraine trade with the president or with two figures that played prominent roles in Trump's alleged campaign to pressure Zelensky: Rudy Giuliani, the president's personal attorney, and Gordon Sondland, the U.S. ambassador to the European Union.

Through September, Lighthizer had nine meetings or phone calls scheduled with Sondland, according to a copy of the trade chief's daily calendar, which is available on the USTR website.

The most recent, a Sept. 10 phone call, came on the same day that Bolton resigned. One day later, amid an escalating controversy over the president's actions toward Ukraine, the administration released the congressionally-approved $391 million in military and security aid.

USTR did not respond to a request for comment.

Almost two months after withdrawing his initial bid to restore Ukraine's trade privileges, Lighthizer in early October sent the necessary paperwork to the White House for a second time. He later withdrew it again, on Oct. 17, as controversy built over the president's Ukraine policy.

Established in 1974, the GSP program permits 120 developing countries to ship a variety of products to the U.S. without paying import tariffs. The program, which requires participating countries to open their markets to American companies, is designed to spur economic development.

The lawmakers want USTR to provide a timeline of "all actions taken by the administration with respect to Ukraine's status as a GSP beneficiary."

USTR did not respond to requests for comment during preparation of The Post's initial story. The administration finally announced the restoration of Ukraine's GSP benefits in a Friday night press release last week, as Washington prepared to host its first World Series game in 86 years.

That action reinstated only $12 million of the $36 million in trade benefits that had been suspended "given continued significant concerns with Ukraine's protection and enforcement of IP rights," USTR said in a statement.

The Democratic senators ask Lighthizer if the president ordered him to tell Ukrainian President Volodymyr Zelensky or other officials about "Trump's desire for assistance in investigating one of his political opponents or their family members or unsubstantiated theories related to Ukraine's involvement in the 2016 U.S. election," according to the letter.

"To ensure that GSP remains an effective and legitimate program that encourages countries to adopt fair trade practices, it is imperative that decisions about beneficiary countries' eligibility follow a transparent, accountable process that is free from partisan political considerations," Wyden and Menendez write.

The Ukraine furor threatens to engulf Lighthizer as two of the administration’s trade initiatives are at key junctures. A new North American trade deal is moving slowly toward potential congressional approval with the trade boss playing a central role in mobilizing support on Capitol Hill. Lighthizer also is leading complex negotiations with Chinese officials that are expected to produce a partial deal for Trump and Chinese President Xi Jinping to sign as soon as next month.