Revelations that President Trump pressured Ukraine’s new leader to investigate the son of former vice president Joe Biden have focused fresh attention on hundreds of millions of dollars in military aid the administration withheld from the country until last week.

Trump’s actions — which inspired a whistleblower complaint that the administration has refused to share with lawmakers — involve a July 25 phone call between the two leaders, according to people familiar with the exchange, one of whom said Trump did not raise the issue of military aid during the conversation.

Privately, congressional Democrats are questioning whether the aid, which remained frozen during Trump’s call with Ukrainian President Volodymyr Zelensky and for several weeks afterward, was related to the “promise” that sparked the whistleblower complaint and what Intelligence Community Inspector General Michael Atkinson told lawmakers constituted an “urgent” and “credible” threat.

During August and September, lawmakers were engaged in what Republican and Democratic aides described as an unprecedented struggle with the administration to release nearly $400 million in military assistance for Ukraine — $250 million of it controlled by the Pentagon and $141 million by the State Department.

In the five years since Congress began approving significant support for Ukraine’s fight with Russian-backed separatists, lawmakers and the White House have periodically clashed over how much and what kind of assistance is appropriate. President Barack Obama controversially resisted bipartisan efforts to send lethal military assistance to Ukraine, and Congress has steadily appropriated more money for those programs than the Obama and Trump administrations requested.

But never before has the White House withheld the entire military allotment for Ukraine until the last days of the fiscal year, as Trump had done, nor has the president ever done so in such an opaque and un­or­tho­dox manner, according to House and Senate aides with knowledge of the process.

According to those aides, who spoke on the condition of anonymity, lawmakers were informed in the spring that the administration intended to start sending money to Ukraine on a rolling basis. But the funds were never transferred — and by August, congressional appropriators became aware that officials at the Pentagon and the State Department had lost control over the process to the Office of Management and Budget, an executive office run by Mick Mulvaney, who also serves as the president’s acting chief of staff.

For weeks thereafter, congressional aides said, the OMB announced short-term holds on Ukraine funds. There was never an express reason given except that the administration was conducting a vague “interagency review” of the funds.

As the holds continued into September, Democrats introduced two amendments: one to ensure they could roll the current fiscal year’s funds into 2020 and another to hold back $5 billion of the defense budget until Trump releases next year’s military aid to Ukraine.

The government’s fiscal calendar begins Oct. 1 and ends Sept. 30.

“The administration asked for the money, was given the money and then refused to spend the money until last night . . . with only 18 days left in the fiscal year,” Sen. Richard J. Durbin (D-Ill.) said during an appropriations committee hearing last week, asking: “Why the delay?”

Privately, Democrats are wondering whether the Trump administration was trying to use that money as leverage over the Ukrainian president, hopeful it would spur an investigation of Biden, who is a leading Democratic candidate for president. Biden on Friday suggested there could be a connection.

“If these reports are true . . . it means he used the power and resources of the United States to pressure a sovereign nation — a partner that is still under direct assault from Russia — pushing Ukraine to subvert the rule of law in the express hope of extracting a political favor,” the former vice president said in a statement.

Congressional Republicans have bristled at any criticism of Trump’s management of the Ukraine money, arguing that he had every right to withhold the money while Ukraine was transitioning to a new president, who had up until his election earlier this year been a comic actor with no political experience. They have also alleged that Obama and Biden tried to leverage U.S. aid in their dealings with Ukraine.

House Republican leader Rep. Kevin McCarthy (R-Calif.) told reporters Friday: “I just watched our former vice president, in his own words, say he influenced Ukraine to fire somebody based upon aid that America was providing.” McCarthy was referring to comments Biden made at the Council on Foreign Relations in 2018, bragging that he had threatened to withhold $1 billion in loan guarantees, not military aid, unless Ukrainian leaders promptly fired the country’s top prosecutor.

“It is interesting to me that the last administration didn’t give Ukraine any military aid [and] we actually have an administration that wants to do that,” Rep. Tom Cole (R-Okla.) said, erroneously.

Obama resisted approving lethal military aid for Ukraine but distributed about $231 million of the $250 million Congress appropriated for military assistance in fiscal 2016, according to congressional appropriators. That was the first year Congress expressly indicated in its defense bill that some of Ukraine’s military assistance — $50 million — was intended for lethal aid.

Though Obama’s stance on lethal aid for Ukraine sparked open protest from senior administration advisers, Democratic and even some Republican aides resisted making a comparison that suggested Trump’s policies were “better” for Ukraine, as he and his political allies have.

When Congress began appropriating large sums of military aid for Ukraine, some criticized Obama for worrying that providing the lethal assistance Kiev wanted might provoke Russia. But there were legitimate and widespread concerns, aides said, that the fledgling Ukrainian armed forces — which at that point were more a shifting consortium of militias organized under the country’s interior ministry than a cohesive, traditional military — would not be able to manage sophisticated weapons systems responsibly.

Furthermore, aides noted, while the Obama administration could have taken bolder steps on military aid, it was clear that the president stood with Ukraine in opposition to Russia — by his statements, his sanctions and his other efforts to assist the country along with European allies. By comparison, aides said, Trump’s disdain for NATO and embrace of Russian President Vladi­mir Putin have made his policy convictions regarding Ukraine less clear.