WASHINGTON — President Donald Trump said Sunday that he will "demand" this week that the Justice Department open an investigation into whether the FBI infiltrated his presidential campaign for political purposes and whether any demands or requests for such action originated with the Obama administration.

Trump tweeted: "I hereby demand, and will do so officially tomorrow, that the Department of Justice look into whether or not the FBI/DOJ infiltrated or surveilled the Trump Campaign for Political Purposes - and if any such demands or requests were made by people within the Obama Administration!"

I hereby demand, and will do so officially tomorrow, that the Department of Justice look into whether or not the FBI/DOJ infiltrated or surveilled the Trump Campaign for Political Purposes - and if any such demands or requests were made by people within the Obama Administration! — Donald J. Trump (@realDonaldTrump) May 20, 2018

The Justice Department declined to comment.

Trump has been promoting a theory circulating in conservative circles about a possible FBI spy on the campaign, though his attorney has cast doubt on it.

Rudy Giuliani, who represents Trump in the ongoing special counsel's investigation into Russian meddling in the 2016 presidential election, said in a television interview last week that neither he nor Trump knows for certain if there was a spy on the campaign. Giuliani said they had been told of "some kind of infiltration."

In any event, the Justice Department's internal watchdog is already examining Republican complaints of FBI misconduct in the early stages of the Russia investigation.

Inspector General Michael E. Horowitz announced an investigation in March at the request of Attorney General Jeff Sessions and congressional Republicans. Sessions and the lawmakers had urged Horowitz to review whether FBI and Justice Department officials abused their surveillance powers by using information compiled by Christopher Steele, a former British spy, and paid for by Democrats as part of the basis to justify monitoring Carter Page, a former campaign adviser to Trump.

Horowitz said his office will look at those claims as well as communications between Steele and DOJ and FBI officials.

Trump did not elaborate on the promised "demand," which he included in one of a series of tweets he sent throughout the day Sunday. On Saturday, Trump tweeted, "If the FBI or DOJ was infiltrating a campaign for the benefit of another campaign, that is a really big deal." He said only the release or review of documents the House Intelligence Committee is seeking from the Justice Department "can give conclusive answers."

The White House did not immediately respond to a request for comment.

Rep. Adam Schiff of California, the senior Democrat on the House Intelligence Committee, called Trump's claim of an embedded spy "nonsense."

"His 'demand' DOJ investigate something they know to be untrue is an abuse of power, and an effort to distract from his growing legal problems," Schiff said on Twitter. "Never mind that DOJ has warned that lives and alliances are at risk. He doesn't care."

The GOP-led House Intelligence Committee closed its Russian meddling probe last month, saying it found no evidence of collusion or coordination between Trump's campaign and Russia, which special counsel Robert Mueller is looking into. Schiff and other committee Democrats were furious and argued that Republicans had not subpoenaed many witnessed they considered essential to the committee's work.

Trump last week accused the Justice Department of trying to frame him by planting a spy in his 2016 campaign. Trump quoted Fox Business anchor David Asman and tweeted Friday: "Apparently the DOJ put a Spy in the Trump Campaign. This has never been done before and by any means necessary, they are out to frame Donald Trump for crimes he didn't commit."

But when asked whether there was an informant on the campaign, Giuliani told CNN, "I don't know for sure, nor does the president, if there really was one," though he said they have long been told there was "some kind of infiltration."

Earlier this month, the National Review raised the question of a possible FBI spy in Trump's campaign. The article cites work by Republican Rep. Devin Nunes of California, an ardent Trump supporter and chairman of the House Intelligence Committee, who has demanded information on an FBI source in the Russia investigation. Opponents of Nunes' request have stressed the need to protect intelligence sources and methods.

The New York Times reported separately last week that at least one government informant met several times with Page and George Papadopoulos, another former foreign policy adviser on Trump's campaign. The Times reported, citing current and former FBI officials, that the informant talked to Page and Papadopoulos because they had suspicious contacts linked to Russia.

Papadopoulos was charged last year in Mueller's investigation and pleaded guilty to lying to the FBI. He has agreed to cooperate with prosecutors.

Sunday was not the first time that Trump accused his predecessor of politically motivated activity against him.

Without substantiation, Trump tweeted in March 2017 that former President Barack Obama had conducted surveillance the previous October at Trump Tower, the New York skyscraper where Trump ran his campaign and transition and maintains a residence. Then-FBI Director James Comey later testified to Congress that internal reviews found no information to support the president's tweets. Trump later fired Comey over the bureau's Russia investigation.