WASHINGTON (AP) — Senate Judiciary Committee Chairman Lindsey Graham said Thursday his committee is opening a wide-ranging investigation into the origins of the Russia investigation, but rejected President Donald Trump's call to bring in former President Barack Obama to testify.

"I am greatly concerned about the precedent that would be set by calling a former president for oversight,'' said Graham, a South Carolina Republican and staunch Trump ally. "No president is above the law. However, the presidency has executive privilege claims against other branches of government.''

Graham noted the surprising nature of his announcement, saying: "To say we are living in unusual times is an understatement."

The U.S. has a sitting president accusing the former president "of being part of a treasonous conspiracy to undermine his presidency,'' Graham said. "We have the former president suggesting the current president is destroying the rule of law" by dismissing a case against Trump's first national security adviser, Michael Flynn. "All of this is occurring during a major pandemic."

The Judiciary Committee will first delve into the Justice Department's decision to dismiss its prosecution of Flynn, as well as actions by the Obama administration to view Flynn's name in intelligence reports during the Russia probe, Graham said.

"We must determine if these requests were legitimate,'' Graham said, referring to requests by top Obama administration officials to "unmask" Flynn's name. The requests are common, including during the Trump administration, which has made thousands of "unmasking" requests.

Graham also said the committee will look into potential abuses of the Foreign Intelligence Surveillance Act, or FISA, during a probe of former Trump campaign adviser Carter Page. The FBI identified Page during the early days of its investigation of Trump's 2016 campaign and secretly targeted his electronic communications.

A federal watchdog later concluded that the FBI made significant errors and omissions in applications it made to a U.S. foreign intelligence court for the authorization to eavesdrop on Page. Those mistakes prompted internal changes within the FBI and spurred a congressional debate over whether the bureau's surveillance tools should be reined in.

"My goal is to find out why and how the system got so off the rails,'' Graham said.

The Judiciary Committee also will look at whether Robert Mueller should have been appointed as special counsel in the Russia probe. The decision to appoint Muller was made in 2017 by then-Deputy Attorney General Rod Rosenstein after Trump fired FBI Director James Comey.

"Was there legitimate reason to conclude the Trump campaign had colluded with the Russians?" Graham asked.

Graham's announcement comes as Trump and his GOP allies begin a broad election-year attack on the foundation of the Russia investigation, including declassifying intelligence information to try to place senior Obama administration officials under scrutiny for routine actions.

The effort has been aided by the Justice Department decision to dismiss the Flynn prosecution, essentially rewriting the narrative of the case in a way that former federal law enforcement officials say downplays the legitimate national security concerns they believe Flynn's actions raised. Flynn pleaded guilty to lying to the FBI about conversations with the Russian ambassador to the United States during the presidential transition period.

Trump and his Republican allies are pushing to reframe the Russia investigation as a "deep state" plot to sabotage his administration, setting the stage for a fresh onslaught of attacks on past and present Democratic officials and law enforcement leaders.

Senate Democratic leader Chuck Schumer railed against Republicans' renewed focus on Flynn and the Russia investigation.

"We're in the middle of a public health and economic crisis, and Senate Republicans are diving head-first into the muck, pursuing diversionary, partisan conspiracy theories to prop up President Trump when President Trump should be focusing on solving this crisis,'' Schumer said.

Hearings by the Judiciary Committee will start in early June, Graham said.

Trump tweeted Thursday that Graham should call Obama to testify. "Do it @LindseyGrahamSC, just do it,'' Trump tweeted. "No more Mr. Nice Guy. No more talk!''

Both Trump and Obama are welcome to come before the committee "and share their concerns about each other,'' Graham said. "If nothing else it would make for great television. However, I have great doubts about whether it would be wise for the country."