Rod J. Rosenstein, Trump’s nominee for deputy attorney general, would be responsible for probes related to the Trump campaign. (Sarah L. Voisin/The Washington Post)

Republican and Democratic leaders on Capitol Hill positioned themselves for a new round of fighting over President Trump’s alleged ties to Russia, with Senate Democrats preparing to wage a fierce battle at a Tuesday confirmation hearing that has suddenly taken on far-reaching implications and a top Republican senator questioning whether the FBI played politics against Trump.

Democrats are poised to aggressively grill Rod J. Rosenstein, Trump’s nominee for deputy attorney general, over how the Justice Department will investigate Russia’s alleged interference in the 2016 election and suspected ties between Trump aides and Russia.

As the second-in-command at the Justice Department, Rosenstein would assume responsibility for probes related to the Trump campaign now that Attorney General Jeff Sessions has recused himself.

Democrats say their concerns about the Trump team’s links to Russia deepened in the wake of Trump’s allegation that President Barack Obama’s administration wiretapped his phones last fall at his Trump Tower headquarters.

“These latest bizarre claims about wiretapping raise the urgency and stakes for a special prosecutor,” Sen. Richard Blumenthal (D-Conn.) said in an interview Monday. “You know, whatever the motive, an effort to distract or to defer the ongoing investigative efforts — there is now unquestionably a need for impartial, objective aggressive leadership of a criminal investigation.”

(Bastien Inzaurralde/The Washington Post)

If Rosenstein does not endorse Democratic calls for a special prosecutor to look into any ties between Trump and Russia, Blumenthal said, “I will use every tool available to block his nomination.”

Speaking on the Senate floor, Minority Leader Charles E. Schumer (D-N.Y.) said, “If he won’t appoint a special prosecutor, he’d need a darn good reason, and it’s hard for me to see one right now.”

But Democrats have very little power to block Rosenstein’s confirmation, and there is no sign Republicans are willing to stand in the way. Furthermore, Rosenstein was confirmed as U.S. attorney by voice vote in 2005 and has been praised by both Democrats and Republicans.

Some Republicans are concerned about revelations that the FBI considered paying a British spy as part of an inquiry into Russian efforts to influence the 2016 election. On Monday, Senate Judiciary Committee Chairman Charles E. Grassley (R-Iowa) asked FBI Director James B. Comey to provide details of such a plan, saying it“raises substantial questions about the bureau’s independence, as well as the Obama administration’s use of law enforcement and intelligence agencies for political ends.”

The letter from Grassley came after Comey asked the Justice Department this weekend to issue a statement refuting Trump’s unsubstantiated claim that Obama ordered a wiretap. The department did not issue such a statement.

(Monica Akhtar/The Washington Post)

Monday’s developments signaled a new and politically charged chapter in the ongoing fight between the two parties over lingering questions about the intelligence community’s contention that Russia meddled in the 2016 campaign — allegations that have emerged as a focal point in the early weeks of Trump’s presidency.

Trump has requested that as part of a look into Russian meddling, congressional intelligence committees should explore whether “executive branch investigative powers were abused in 2016,” White House press secretary Sean Spicer said Sunday.

Grassley’s letter to Comey raised questions about the FBI’s investigation of Russian meddling in the 2016 election. The spy, Christopher Steele, became well known in January after a dossier he put together on behalf of Trump’s political opponents became public.

Neither the FBI nor Steele responded to requests for comment Monday.

Last week, The Washington Post reported that the FBI viewed Steele as credible and, weeks before the election, agreed to pay him to continue looking at unproven allegations of improper coordination with Russia.

The Steele dossier cited sources claiming that Trump and his associates had ties to Russia — and linked some of those associates to Russian efforts to influence the 2016 campaign. Among other things, it cited sources alleging that Trump associates colluded with the Kremlin on cyberattacks on Democrats and that the Russians held compromising material about Trump.

These and other claims have not been verified, and they have been emphatically denied by Trump and his allies.

But Democrats are expected to step up their calls for a special prosecutor at the Rosenstein hearing Tuesday as they seek an aggressive probe into all Russia-related charges.

If confirmed, Rosenstein is expected to take over responsibilities managed by acting deputy attorney general Dana Boente, who is currently handling Trump-related matters because of Sessions’s recusal. He would also have the power to appoint a special prosecutor if he wished.

Congressional Democrats have made clear that Sessions’s recusal is not enough to satisfy them, noting that a deputy attorney general would also potentially be susceptible to the president’s influence, particularly because the president retains the authority to fire any official who crosses him.

Schumer formally requested Monday that Justice Department Inspector General Michael Horowitz “quickly and thoroughly” investigate political interference in Russia-related probes.

In a detailed letter to Horowitz, Schumer laid out questions that are likely to continue to dominate Democratic discourse. But the scope of Schumer’s questions would probably be rebuffed by the White House, which would likely invoke executive privilege to avoid questions about private conversations Trump has had with top aides.

Rep. Jim Himes (D-Conn.), a member of the House Intelligence Committee, said Monday that he has seen no evidence to support Trump’s claim of a wiretap. He pointed out that a president cannot order a wiretap and said that if the Justice Department legally obtained one, it could be a sign of highly questionable behavior by Trump during the campaign.

“That judgment would have been made by a federal judge,” Himes said on MSNBC. “And that, of course, points to the possibility, if any of that is true, that there is probable cause for law enforcement to say that there is something wrong at Trump Tower.”

A wiretap to gather foreign intelligence must be approved by a Foreign Intelligence Surveillance Act court to show that there is probable cause.

Trump’s allegation sent Republican lawmakers scrambling at the beginning of a pivotal legislative week. GOP leaders are hoping to show progress on their long-stated goal of repealing and replacing the Affordable Care Act.

In his letter to Horowitz, Schumer asked the inspector general to explore key questions.

“Has the investigation into Russia been compromised by political interference?” Schumer asked. “Have there been any attempts by any White House official to interfere with the investigation? Did Attorney General Sessions, who is now recused from this investigation, or his close associates personally manage the work of career officials at DOJ or the FBI in the course of the investigation thus far?”

Schumer wants Horowitz to determine whether Trump and Sessions discussed the dismissal of former national security adviser Michael Flynn and whether the president and attorney general discussed the ongoing Russia inquiries. The letter also seeks to determine whether career Justice Department officials have “been the subject of pressure from the White House” regarding the investigations.

John Lavinsky, a spokesman for Horowitz, declined to comment on Schumer’s letter.

Congressional Republicans have shown a willingness to look at Trump’s claims about wiretapping. But many have also distanced themselves from his charges.

“Thus far, I have not seen anything directly that would support what the president has said,” Rep. Jason Chaffetz (Utah), the chairman of the House Oversight Committee, said on CBS News.

Chaffetz said it is “premature” to conclude that there is “no backing evidence” to validate Trump’s allegation, which the president delivered on Twitter over the weekend and which Obama’s office has denied. Speaking to the Fox News Channel, Chaffetz said Congress will take a look at the matter and that “the Obama administration’s been notorious on this type of stuff.” But he also raised the possibility that Trump’s accusation may be baseless.

Senate Intelligence Committee Chairman Richard Burr (R-N.C.) said his panel “will follow the evidence where it leads, and we will continue to be guided by the intelligence and facts as we compile our findings.”

Sen. John McCain (R-Ariz.) said in an interview that if there was “no basis” for Trump’s charges, there was “no reason for an investigation.”

“First, the American people need to know what evidence he has to make such a charge,” McCain said. “I’m all for a congressional investigation, but the president said it based on something. What was the basis of his conclusion that [Obama] broke the law by wiretapping Trump Tower?”

Sari Horwitz, Ellen Nakashima, Ed O’Keefe, Abby Phillip and Elise Viebeck contributed to this report.