President Trump and his Republican allies are rapidly shifting their focus to former vice president Joe Biden’s son Hunter, reviving attacks that led to Trump’s impeachment, in an effort to broadly define as corrupt the potential Democratic presidential nominee.

As Biden surged this week in the Democratic nominating contest — and with exit polls from Super Tuesday’s primaries showing he has captured at least some of the white working-class voters that propelled Trump’s 2016 victory — the president vowed to make Hunter Biden’s work in Ukraine a “major issue” in the general election, should Joe Biden win the nomination.

Trump is getting assistance from Republicans in Congress, some of whom are trying to revive an investigation pertaining to Burisma, the Ukrainian company that hired Hunter to serve on its board, and to open new probes that could uncover information helpful to the president’s reelection campaign.

Joe Biden and his aides are bracing for an onslaught of attacks from Republicans over Hunter — and are planning to mount an assault of their own on what they see as Trump’s self-dealing and on his placing family members in senior White House positions. They also plan to highlight the president’s refusal to release his tax returns.

Trump’s congressional backers are seeking to breathe new life into the Biden-Ukraine allegations, which have been dormant since the president’s impeachment. House Minority Leader Kevin McCarthy (R-Calif.) speculated Thursday about Biden’s son as well as his brother, and he criticized Democratic committee chairmen for not opening probes into their activities.

“What did they do while he was vice president?” McCarthy asked reporters. “What happened to Burisma? . . . I think there’s a real question out there from the American public.”

McCarthy denied that this investigative curiosity was influenced by presidential politics. His comments came less than 24 hours after Trump phoned in to his friend Sean Hannity’s Fox News Channel show and accused the Biden family of being “purely corrupt.” The president said he would go after the Bidens far more aggressively in a general election than the other Democratic candidates have in the primaries.

“The other people that are running, trying to beat him, don’t bring it up,” Trump said. “That will be a major issue in the campaign. I will bring that up all the time, because I don’t see any way out. I don’t see any way — for them, I don’t see how they can answer those questions.”

Tim Murtaugh, the communications director for Trump’s reelection campaign, argued that Biden already has been damaged by “the Hunter Biden-Burisma saga” and said the matter could be a vulnerability for Republicans to exploit in the general election. Murtaugh said the former vice president needs to answer, “What exactly did the Ukrainians think they were getting for their money?”

Hunter, 50, served for nearly five years on the board of Burisma, Ukraine’s largest private energy company, whose owner came under scrutiny by Ukrainian prosecutors for possible abuse of power and unlawful enrichment. Hunter was not accused of any wrongdoing in the investigation.

As vice president, Biden pressured Ukraine to fire the top prosecutor, Viktor Shokin, who Biden and other Western officials said was not sufficiently pursuing corruption cases. At the time, the investigation into Burisma was dormant, according to former Ukrainian and U.S. officials.

Biden allies have noted that Republicans were largely quiet about Hunter and Burisma while Biden’s campaign floundered in recent months and only picked up their scrutiny in recent days as Biden began winning primaries.

Biden’s campaign has long heard the drumbeat of Trump’s criticism. From the moment Biden launched his campaign last spring, Trump has attacked him on Twitter and sought to shine a spotlight on Hunter. At times, Biden advisers say, the whole enterprise has felt like a rapid-response operation.

One major lesson that Biden’s advisers took from the past several months is the benefit of being aggressive and attacking the credibility of Trump’s allies.

Andrew Bates, a Biden campaign spokesman, said his team was prepared to fight back on any Ukraine-related attacks.

“Any time this conspiracy theory is raised by the President or Senators who disrespect their office enough to transform their committees into craven arms of his reelection campaign, we will hammer — with a baseball bat — the fact that Donald Trump is hands down the most corrupt American president of all time,” Bates said in a statement. “If they really want to go there, they should buckle up.”

Polls have long showed Biden defeating Trump in head-to-head matchups despite a year of withering GOP attacks. The former vice president’s advisers argued that his name recognition and favorability ratings could inoculate him — and even some Republican politicians have said that they generally like Biden.

Indeed, Republican pollster Frank Luntz said attacks on Hunter may not prove effective for Trump.

“I don’t think going after the child of a vice president is going to move any undecided voter,” Luntz said. “It’s just not top of mind, and it doesn’t rank in the 10 priorities that an undecided voter cares about.”

Fact-checkers have debunked Trump’s attacks related to Hunter’s work in Ukraine, and some Trump administration officials testified under oath during the impeachment inquiry that Biden did not do anything wrong.

It does not appear that Biden’s campaign will modify how it handles the attacks, and Hunter continues to operate independently of the campaign. When Biden celebrated Tuesday night’s election results — one of the most triumphant moments of his political career — with some members of the family in Los Angeles, he was not joined onstage by Hunter, who lives nearby with his new wife.

Trump advisers and allies see Hunter as central to an argument they plan to make that Biden is corrupt and used his decades in public service to enrich his family. The argument is similar to one Trump advanced against Hillary Clinton in 2016.

“I would expect us to be hearing a whole heck of a lot more about Hunter — and not just Hunter, but the Biden family self-enrichment game and why that means Joe Biden has the same problems that Hillary Clinton had,” said Jason Miller, a former Trump campaign adviser who co-hosts a pro-Trump podcast and radio show.

Josh Schwerin, a senior strategist at Priorities USA, a Democratic super PAC that is preparing to help the party’s eventual nominee, questioned whether this could again be an effective argument for Trump.

“A big difference from 2016 is that Trump is now on the defensive on corruption,” Schwerin said. “He was just impeached, and there has been a constant flow of reporting on the corruption, chaos and scandals of the Trump administration.”

Unite the Country, a pro-Biden super PAC formed last fall to help bolster Biden against Trump’s attacks, is also poised to play a role. Advisers there are focused on ensuring that Biden wins primary states such as Michigan and Florida, but they are also beginning to contemplate their role in a general election counteroffensive.

In addition, a super PAC funded by former New York mayor Mike Bloomberg, who withdrew from the race and endorsed Biden on Wednesday, could play a major role.

“You can’t probably stay above it all,” said Rep. Debbie Wasserman Schultz (D-Fla.), who has endorsed Biden, adding that Trump would “use every unethical tactic in the tool belt.”

It is unclear how the Biden campaign would handle attacks on Hunter that go beyond the dealings in Ukraine that were part of the impeachment hearings. Some close to Biden think that personal attacks on his substance abuse problems, or his out-of-wedlock child, could end up backfiring.

Trump also risks inviting scrutiny of his own adult children. Trump’s sons Eric and Donald Jr. run the Trump Organization, which has charged the U.S. government to house the government employees who travel with the president — at rates as high as $650 a night for hotel rooms at Trump’s Mar-a-Lago Club in Florida, and $17,000 a month for a cottage at Trump’s club in Bedminster, N.J. The Secret Service alone has paid Trump’s company more than $628,000 since Trump took office, according to receipts obtained through public-records requests.

Trump properties have also seen a huge increase in business from Republican candidates and campaign committees since Trump was elected in 2016. The company’s properties have been paid $5.8 million by candidates and PACs since the start of 2017.

Bates, the Biden spokesman, said, “Donald Trump’s company is actively leeching millions off of American taxpayers as we speak. He is now in year five of breaking his promise to the American people to release his tax returns. And he is the only American president who has ever tried to force a foreign country to lie about the candidate he’s too afraid to look in the eye.”

On Capitol Hill, interest in Biden is rising along with the candidate’s fortunes. The most significant congressional probe targeting the Bidens is an outgrowth of a 2015 Clinton email investigation and is being directed by Sen. Ron Johnson (R-Wis.), the chairman of the Senate Homeland Security and Governmental Affairs Committee.

Johnson requested State Department documents pertaining to Hunter and Burisma last November, and last month, he requested Secret Service documents regarding Hunter’s travel during his father’s vice presidency. On Feb. 24, Johnson notified the top Democrat on his panel, Sen. Gary Peters (Mich.), that he intended to issue a subpoena for testimony from former Ukrainian diplomat Andrii Telizhenko, who is tied to Burisma.

Peters challenged the subpoena, and Johnson is planning a March 11 vote on the demand. All of the committee’s Democrats are expected to oppose it, while Sen. Mitt Romney (R-Utah) signaled misgivings Thursday.

“I would prefer that investigations are done by an independent, nonpolitical body,” Romney said. “There’s no question the appearance is not good.”

A second Republican on the panel, Sen. Rob Portman (Ohio), said he is undecided.

Johnson rejected accusations of political motivations, saying in an interview, “I see no reason why anybody should object to getting this very narrow set of records.”

Senate Majority Whip John Thune (S.D.), the chamber’s No. 2 Republican, defended the inquiry into Hunter’s work in Ukraine as a “fair matter of oversight.”

“It’s going to be the grist of the campaign,” Thune said. “There’s no question about it. … As you debate presidential elections, people’s backgrounds are obviously a part of that.”

David A. Fahrenthold and Paul Kane contributed to this report.