Whether President Trump withheld military aid to Ukraine to force President Volodymyr Zelensky to investigate the Bidens has emerged as the linchpin of Democrats’ impeachment drive.

Mr. Trump says he held up the $250 million aid package because European nations were not coughing up enough money, even as Ukraine served as a literal “wall” between them and Russia. But he relented after Republican allies convinced him to look beyond his pocketbook concern, he said.

“I gave the money because [Sen.] Rob Portman and others called me and asked. But I don’t like to be the sucker. And European countries are helped far more than we are, and those countries should pay more to help Ukraine,” Mr. Trump said at a press conference with Finnish President Sauli Niinisto.

So far, his story is holding up.

Mr. Portman, co-chairman of the Senate Ukraine Caucus, says the conversation in question occurred Sept. 11 and that former Vice President Joseph R. Biden didn’t come up. Nor did former New York City mayor and Trump lawyer Rudolph W. Giuliani, cited by a whistleblower as a “central figure” in Mr. Trump’s alleged efforts to get foreign nations to investigate his 2020 Democratic rival.

“Rob has long been Ukraine’s staunchest supporter as it works to combat Russian aggression and strengthen its democracy. He has continually advocated for and helped secure the security assistance they need, and when there was a hold on the funds, he acted to ensure they were released,” said Portman spokeswoman Emily Benavides. “At no time during his work advocating for Ukraine did the former vice president or former mayor of New York come up. Rob is the one who helped secure the release of the bipartisan-agreed upon money, which the president confirmed yesterday.”

The money, which had been frozen for weeks, was released the day after the Portman-Trump call with $141 million more in State Department assistance, thrilling Ukraine supporters in Congress but leaving questions about the delay.

Although the aid package, which includes lethal weapons, such as Javelin anti-tank missiles, was previously approved by Congress, the White House unexpectedly ordered Defense Secretary Mark T. Esper and then-National Security Adviser John R. Bolton to review the program earlier this year. The Pentagon determined the package was in the U.S. national interest, so Democrats say there has to be more to the story.

Mr. Trump put a hold on the aid package in early to midsummer with little explanation. While in limbo, Mr. Trump held his infamous call with the Ukrainian TV comedian-turned-politician in which he asked Mr. Zelensky to look into Mr. Biden and his son Hunter, who had business ties in Ukraine.

Mr. Trump also queried about the location of a hacked Democratic National Committee hacked server from 2016, couching it as a “favor.”

While the president noted all the support the U.S. gives Ukraine, there wasn’t an explicit quid pro quo involving the aid money on the call, prompting cries of a “nothingburger” from Mr. Trump’s allies.

Whether Democrats can prove that Mr. Trump was holding out on Ukraine to gather dirt on the Bidens may determine whether the House Democrats’ impeachment drive sputters or turns into a full-boiled scandal that forces Republicans to bail on Mr. Trump.

“The aid piece is important to the impeachment debate because it relates to U.S. support of an important ally and whether Trump used Ukraine’s need for military assistance to extort help with the Biden probe,” said Darrell West, director of governance studies at the Brookings Institution. “It turns what is a major violation into an impeachable offense. The use of military assistance for partisan political purposes elevates Trump wrongdoing to a much higher level.”

Seven freshmen House Democrats with national security backgrounds have labeled it an “impeachable offense” if Mr. Trump did withhold military aid to Ukraine to try to get the country to dig up dirt on his political opponent.

Sen. Chris Murphy, a Connecticut Democrat on the Senate Foreign Relations Committee, has called the White House’s decision to release held-up aid to Ukraine as the whistleblower issue surfaced “incredibly suspicious.”

Within two days of the aid release, House intelligence committee Chairman Adam B. Schiff issued a subpoena for acting Director of National Intelligence Joseph Maguire, saying the information of the July 25 phone call was being inappropriately withheld.

Mr. Maguire said it was not released because of a specific interpretation of the law, which says that because the complaint involves the executive branch, the legislative branch does not have jurisdiction over the matter.

Mr. Biden, meanwhile, says he won’t let allegations of corruption against him and his family deter his quest for the 2020 Democratic nomination. He is echoing House Democrats who believe Mr. Trump tried to “extort the existential fears of Ukraine’s leaders for political advantage.”

“Now we know that he may have held up political support and vitally needed aid appropriated by Congress to pressure Ukraine to give-in to his political demands,” Mr. Biden said in Reno, Nevada.

The former vice president said that “abuse of power is the defining characteristic of the Trump presidency” and that he stands behind House Democrats’ impeachment inquiry.

No president in modern history has been the target of impeachment in his first term heading into a reelection campaign. President Bill Clinton was impeached in his second term by the House but not removed from office by the Senate.

President Richard Nixon resigned in his second term while facing certain impeachment. Impeachment is the equivalent of a political indictment and can be approved with a majority vote in the House. If articles of impeachment are approved, then the Senate holds a trial. It takes a two-thirds vote in the Senate to convict and remove the president.

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