Former Deputy Attorney General Rod Rosenstein testifies before a Senate Judiciary Committee hearing on Capitol Hill in…
Former Deputy Attorney General Rod Rosenstein testifies before a Senate Judiciary Committee hearing on Capitol Hill in Washington, June 3, 2020.

Former U.S. Deputy Attorney General Rod Rosenstein defended his decision to open a special counsel investigation into Russian interference in the 2016 election, telling a Senate panel Wednesday that while there was "reasonable suspicion" for the probe, he should not have signed an application for a surveillance warrant against a Trump campaign adviser.

The Senate Judiciary Committee called Rosenstein in to testify about the origins of the FBI investigation into Russian interference in the 2016 election, the first step in a broader effort by the Republican-majority U.S. Senate to investigate alleged abuses on the part of Obama administration officials, as well as presumptive 2020 Democratic nominee, former Vice President Joe Biden.

Chair of the Republican-led Senate Judiciary Committee Lindsey Graham of South Carolina prepares to hear testimony from former Deputy Attorney General Rod Rosenstein in the Dirksen Senate Office Building in Washington, June 3, 2020.

Judiciary Committee Chairman Lindsey Graham is set to issue 53 subpoenas Thursday, in an attempt to compel testimony from former Obama administration officials, including former FBI Director James Comey and former Attorney General Loretta Lynch. Last month, the Senate Homeland Security Committee approved a subpoena for Blue Star Strategies, the consulting firm that employed Biden's son, Hunter Biden.

The investigation by Senate Republicans — coming five months before the 2020 presidential election — revives President Donald Trump's long-standing claims there was a conspiracy within the Obama administration to hinder his bid for the presidency and later, his incoming administration.

"There are millions of Americans pretty upset about this," Graham said in his opening statement. "There are people on our side of the aisle who believe that this investigation, Crossfire Hurricane, was one of the most corrupt, biased criminal investigations in the history of the FBI. And we would like to see something done about it."

A Justice Department investigation found last year that the FBI had committed errors in investigating Russian interference in the 2016 election, including mistakes in an application to put former Trump campaign adviser Carter Page under surveillance. However, the investigation did not find any evidence of political bias by federal investigators, as Trump alleges.

Sen. Mazie Hirono, D-Hawaii, questions former Deputy Attorney General Rod Rosenstein via teleconference during a Senate Judiciary Committee hearing on Capitol Hill in Washington, June 3, 2020.

Rosenstein told lawmakers Thursday he would not have signed the warrant authorizing surveillance of Page under the Foreign Intelligence Surveillance Act (FISA) if he had been aware of the errors in the application.

"Every application that I approved appeared to be justified based on the facts it alleged, and the FBI was supposed to be following protocols to ensure that every fact was verified," Rosenstein said in his opening testimony.

Rosenstein defended the recusal of then-Attorney General Jeff Sessions that put him in charge, calling Sessions one of the most "principled people in Washington." He also described the decision-making process leading to the appointment of Robert Mueller as special counsel, telling lawmakers "appointing a special counsel was the best way to complete the investigation appropriately and to promote public confidence in its conclusions."

Rosenstein left the Justice Department in May 2019 after two stormy years as the deputy attorney and shortly after Mueller had completed his investigation.

Democratic Senator from California Dianne Feinstein prepares to hear testimony from former Deputy Attorney General Rod Rosenstein in the Dirksen Senate Office Building in Washington, June 3, 2020.

Senator Dianne Feinstein, the ranking Democratic member on the Judiciary Committee, noted the Mueller investigation "uncovered more than a 120 contacts between the Trump campaign and individuals linked to Russia."

Feinstein went on to directly quote from the Mueller report, citing its findings that the Trump campaign "expected it would benefit electorally from Russia's interference" and that members of the campaign lied to Mueller and the U.S. Congress about their contacts with Russia.

Democrats have criticized efforts to investigate the Obama administration, saying that Republicans are using congressional resources to help the president's personal political fortunes while there are more pressing issues.

"Those who tuned in might have expected that we'd have a hearing concerning the public health crisis facing America, the pandemic, which we're fighting every day, which has claimed over 100,000 American lives," Democratic Senator Dick Durbin said Wednesday.

"They might even wonder if we would take up the issue of racism in the administration of justice in America," he added. "Certainly a timely topic, but we're not. They might wonder if we would ask a question about President Trump's suggestion 48 hours ago that he would have a federal militarization of law enforcement across the United States. Certainly a significant constitutional issue. But no, not taking that up today. Instead, taking up the Mueller report, an investigation that was completed more than a year ago."

Chairman Graham has announced the committee does plan to hold a hearing on police use of force.