The threat of an expanding special counsel investigation into President Trump’s finances has rattled his conservative base with fears that the New York billionaire could be brought down by shady business deals — not collusion with Russia.

Justice Department special counsel Robert Mueller has widened the scope of the Russia investigation to include Mr. Trump’s business deals, according to a Bloomberg News report.

The president had warned that such a move would cross a line.

“Those transactions are in my view well beyond the mandate of the special counsel, are unrelated to the election of 2016 or any alleged collusion between the Trump campaign and Russia, and most importantly, are well beyond any statute of limitation imposed by the United States Code,” said John Dowd, Mr. Trump’s private attorney.

Moving the investigation into deals going back more than a decade also irked Mr. Trump’s staunchest supporters, though they remain convinced that there was never collusion with Russia and that the special counsel is part of an open conspiracy to remove the president.

The investigation reportedly now includes Mr. Trump’s involvement with Russians in a development in Manhattan’s Soho neighborhood, a Russian oligarch’s purchase of a Trump mansion in Florida in 2008 and the 2013 Moscow staging of the Trump-owned Miss Universe pageant.

“There’s reason to be concerned,” said Tim Selaty Sr., the founder of Citizens for Trump and Tea Party Community, a 170,000-member social media network.

“The fact that they are now considering dipping into the last 12 years of what President Trump is doing — based off of what? What is the reason to go in that direction?” he said. “It’s not going to be good for him, of course.”

The numerous investigations into Russian efforts to influence the election and accusations of Trump campaign collusion include probes by Congress, the FBI and the Justice Department special counsel.

The inquiries, which in the case of the FBI has been ongoing for at least a year, gained momentum in recent weeks with the revelation that Donald Trump Jr. met in June 2016 at Trump Tower with a Russian lawyer who offered dirt on Democratic presidential nominee Hillary Clinton.

The Trump team is preparing to push back against the investigators, according to reports Thursday evening in The New York Times and The Washington Post.

Both newspapers, citing unidentified sources, reported that Mr. Trump’s attorneys are piecing together conflict-of-interest cases against Mr. Mueller’s staff and discussing the scope of the president’s pardon powers.

CNN legal analyst Jeffrey Toobin downplayed the reports Thursday night, noting probes of special prosecutor Ken Starr’s staff during the Clinton years and that lawyers routinely spin out scenarios.

But the investigation into Mr. Trump’s business deals caused more alarm among Trump loyalists.

Mr. Selaty said he still believed Mr. Trump was an honest businessman. But he worried that the investigation had become a search for anything to discredit Mr. Trump, potentially eroding support and fueling efforts by Democrats and news media to force the president out of office.

“We want our president to be honest, but if there are things in his past that are unsavory, we won’t be happy,” said Mr. Selaty. “I’m also doing this to hold Trump accountable and hold his feet to the fire now that he’s been elected.”

Ken Crow, a conservative activist and editor of CrowsNestPolitics.org, said nobody becomes a multibillionaire in business without some “shady deals” along the way.

“I am nervous. I supported this guy lock, stock and barrel. I loved him dearly, and now I’m scared,” he said. “I think the Democrats are going to succeed, and the Republicans aren’t helping him any. It’s almost like they are pouring gasoline on the fire.”

Republican Party strategist Ford O’Connell said the worry at this stage was “vast overreaction.”

“There’s always going to be people who are nervous,” he said. “The best way Trump can overcome these fears is putting some wins on the board” with his legislative agenda on tax cuts and repealing and replacing Obamacare.

Frustrations with the multiple Russia investigations on Capitol Hill also bubbled over Thursday as Senate leaders threatened to subpoena Mr. Trump Jr. and former Trump campaign chairman Paul Manafort if they fail to show up next week to testify about their interactions with Kremlin figures.

Senate Judiciary Committee Chairman Chuck E. Grassley, Iowa Republican, and ranking Democrat Dianne Feinstein of California warned that they would execute the hardball move against some of the president’s closest confidantes amid growing concern about the younger Mr. Trump’s meeting with Russian lawyer Natalia Veselnitskaya.

Mr. Manafort and the president’s son-in-law and special adviser Jared Kushner also attended the meeting.

Mr. Trump Jr. and Mr. Manafort are scheduled to testify before the committee Wednesday, but neither has publicly confirmed an appearance. Mr. Kushner, who had already been under suspicion for other interactions with Russians, is scheduled to testify Monday before a closed-door hearing of the Senate Select Committee on Intelligence.

“Chairman Grassley and ranking member Feinstein have agreed to issue subpoenas, if necessary to secure their testimony,” the committee said in a statement.

This week, the committee canceled a hearing to question the figure who supported the anti-Trump dossier, Washington political operative Glenn R. Simpson.

The request to testify was voluntary, but sources close to Mr. Simpson told The Washington Times that he never intended to show.

He is on the schedule for next week’s hearing with Mr. Manafort and Mr. Trump Jr., but negotiations for his appearance are continuing.

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