WILMINGTON, Del. (AP) — President-elect Joe Biden's historically challenging transition to power is suddenly becoming even more complicated.

A federal investigation into the finances of Biden's son Hunter threatens to embolden congressional Republicans, who have already shown little willingness to work with the incoming president or even acknowledge his clear victory in last month's election. For sure, it will complicate Senate confirmation hearings for Biden's yet-to-be-named attorney general, who could ultimately have oversight of the investigation into the new president's son.

It all raises the prospect of even deeper dysfunction in a capital that is already struggling to address the nation's most pressing crises, including a surging pandemic whose daily death tolls are beginning to surpass the devastation of the Sept. 11 attacks. Republicans, particularly those eyeing presidential runs in 2024, are making clear they will press Biden on the issue.

"Joe Biden needs to pledge today that he will cooperate with the federal investigation and answer any questions under oath," Sen. Josh Hawley, R-Mo., said Thursday, "and that if he is sworn in as president, no federal investigator or attorney working on the Hunter Biden criminal case will be removed."

Hunter Biden has long been a source of worry for his father's campaign and was the subject of repeated unsupported accusations by President Donald Trump and his allies. But news of the probe, which was revealed on Wednesday and scrutinizes some of Hunter Biden's Chinese business dealings and other transactions, caught most of his father's staffers by surprise.

The president-elect had no public appearance Thursday as he moved forward with filling out his administration. But the investigation threatens to destabilize a transition that has prioritized a methodical rollout of Cabinet selections, White House hires and policy goals — all meant to guarantee momentum when Biden takes office and immediately has to grapple with a surging pandemic and shaky economy.

Most notably, the probe casts a spotlight on one of Biden's most important choices: his attorney general.

Alabama Sen. Doug Jones and federal appeals court judge Merrick Garland have emerged as the leading contenders, three people familiar with the matter told The Associated Press this week. But dynamics could shift, as any Biden choice now will be scrutinized for any perceived loyalty to the president-elect and bias in any probe of his son.

Both Garland and especially Jones have longtime ties to Biden.

The president-elect himself is not a subject of the investigation. And Biden aides believe that because other Hunter Biden stories have blown over, this will, too. They note that a tax fraud investigation pales in comparison to Trump's refusal to concede the election or to the pandemic that has killed more than 290,000 Americans.

Biden is expected to announce more Cabinet picks, but not attorney general, on Friday.

Trump's initial public response was surprisingly muted, just a pair of tweets about a Fox News segment on the story Wednesday night. But privately, he demanded to know why the investigation was not revealed ahead of Election Day, accusing officials of deliberately stalling in order to help Biden's chances, according to two Republicans familiar with the conversations but not authorized to discuss them publicly.

Other Republicans, including possible presidential contenders, were anything but shy in piling on.

"If there were ever circumstances that created a conflict of interest and called for a special counsel, I think those circumstances are present here," Sen. Tom Cotton of Arkansas said Thursday. "The Biden family has been trading on Joe Biden's public office for 50 years. Do we really think that that will change if Joe Biden becomes president, the highest office in the land?"

Some of Trump's children have also faced some legal questions.

Donald Trump Jr. was scrutinized in special counsel Robert Mueller's Russia probe for the 2016 meeting he had with a Kremlin-connected lawyer. And Ivanka Trump was recently deposed by investigators from the District of Columbia attorney general's office as part of its lawsuit alleging the misuse of inaugural funds.

The Hunter Biden investigation is the latest in a series of politically charged inquiries that the Justice Department has grappled with in the past five years, following probes into Hillary Clinton's email use and the Trump campaign's ties to Russia.

Regardless of the facts of the investigation, a new Justice Department will likely feel compelled to assert its independence from the White House following allegations that its actions were overly politicized during the Trump administration. Biden has said he will play no role in department investigative decisions.

"There are plenty of people that are going to have their hands in this one, but I think it's really going to be driven by the new administration's push to have total independence on any investigation," said former Justice Department prosecutor Michael Weinstein, a New Jersey defense lawyer.

"I think that's going to be paramount," he added. "They are going to bend over backwards to make sure that they do the contrary to the current administration — which is independence, let the investigation takes its course."

In this case, though, there is no perfect outcome for Biden.

A protracted criminal investigation that results in an indictment would be a major distraction and then some as the new president tries to implement his agenda. But if the Justice Department decides against bringing charges, officials will feel pressure to explain their steps and reassure the American public the inquiry was done in a competent and thorough way.

Federal investigators served a round of subpoenas on Tuesday, including to Hunter Biden, according to a person familiar with the investigation who spoke on condition of anonymity to discuss an ongoing probe.

The federal investigation, centering on potential tax crimes, had been going on at least a year before Biden announced his candidacy. Investigators did not reach out in the weeks prior to voting because of a department policy surrounding elections that prohibits overt investigative acts.

Biden, fiercely protective of his own family, was said to be, in a statement released by his transition, "deeply proud of his son, who has fought through difficult challenges, including the vicious personal attacks of recent months, only to emerge stronger."

Biden struck a similarly defiant tone throughout the campaign whenever questions emerged about his son, which they frequently did. When Trump assailed Hunter Biden as a drug user during the first presidential debate in Cleveland, his father acknowledged that Hunter had had past substance abuse issues but added, "He's worked on it. I'm proud of him. I'm proud of my son."

The spotlight on Hunter Biden intensified in the election's final weeks, after Trump and his attorney Rudy Giuliani pushed unsubstantiated claims about his foreign business dealings. Those were based in part on New York Post reporting on a laptop that supposedly once belonged to Hunter Biden and was abandoned at a Delaware repair shop.

Hunter Biden had joined the board of Ukrainian gas company Burisma in 2014, around the time his father, then the vice president, was helping conduct Obama administration foreign policy with Ukraine. But, in a subsequent report, Senate Republicans did not produce any evidence that the hiring influenced U.S. policies.

Joe Biden largely refused to even acknowledge the laptop controversy. His son, meanwhile, kept almost completely out of the public eye during the nearly 18 months his father was running for president.

One of the few times he was seen was on stage outside the convention center in Wilmington, Delaware, as his father was surrounded by joyous family members following his speech declaring victory in the presidential election. Hunter Biden, and his baby son, were right in the middle.

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Tucker and Weissert reported from Washington. Additional reporting by Associated Press writer Mike Balsamo in Washington.