WASHINGTON (Reuters) - Former Acting U.S. Attorney General Sally Yates said on Monday she warned the White House in January that then-national security adviser Michael Flynn had been compromised and could have been vulnerable to blackmail by Russia. Yates testified at a Senate Judiciary subcommittee hearing that focused primarily on Flynn, and did not shed much light on other aspects of investigations of allegations that Russia meddled in the 2016 U.S. election and whether there was collusion between President Donald Trump’s campaign and Moscow. Yates repeatedly declined to discuss details of the investigation in a public forum. Former Director of National Intelligence James Clapper, who also testified, said he stood by past assertions that he had not seen evidence of such collusion but also declined to comment on classified matters. Yates briefly led the U.S. Justice Department until Trump fired her on Jan. 30 for declining to defend his travel ban on seven Muslim-majority countries. She told White House counsel Don McGahn on Jan. 26, less than a week into Trump’s presidency, that Flynn had not been telling the truth about his contacts with Russia’s ambassador to Washington. Making her first public statements about the issue, Yates said she feared Moscow could try to blackmail Flynn because it also knew he had not been truthful about conversations he had with Ambassador Sergei Kislyak about U.S. sanctions on Russia. Flynn, a retired general once seen as a potential Trump vice president, has emerged as a central figure in the Russian probes. Russia has repeatedly denied any meddling in the election and the Trump administration denies allegations of collusion with Russia.                  Yates told the hearing she had been concerned that “the national security adviser essentially could be blackmailed by the Russians.” “Logic would tell you that you don’t want the national security adviser to be in a position where the Russians have leverage over him,” she said. Trump, who continued to praise Flynn, waited 18 days after Yates’ warning before Flynn’s forced resignation for failing to disclose the content of his talks with Kislyak and then misleading Vice President Mike Pence about the conversations.  Several Democratic senators questioned Trump’s delay. Yates said that in her meetings, McGahn “demonstrated that he understood this was serious. .. If nothing was done, certainly that would be concerning.” During that section of the hearing, Clapper described as accurate a report in the Guardian newspaper that British intelligence officials became aware in late 2015 about suspicious interactions between Trump advisers and Russian agents, and that the information was passed on to U.S. intelligence agencies. “Yes, it is (accurate), and it’s also quite sensitive,” Clapper said. Yates was a holdover from the administration of President Barack Obama. Obama had warned Trump, then president-elect, not to give the post of national security adviser in his administration to Flynn just after the Republican’s surprise victory in the Nov. 8 election, a former Obama aide said.     The warning, first reported by NBC News, came up during a discussion of White House personnel. White House spokesman Sean Spicer told reporters Obama had communicated concerns about Flynn. It “shouldn’t come as a surprise to anyone, given that General Flynn had worked for President Obama, was an outspoken critic of President Obama’s shortcomings,” Spicer said.     Obama pushed Flynn out in 2014 from his job as director of the military’s Defense Intelligence Agency, or DIA. Congressional committees began investigating after U.S. intelligence agencies concluded that Russian President Vladimir Putin ordered hacking of Democratic political groups to try to sway the election toward Trump. The main investigations are being conducted by congressional Intelligence Committees, although Democrats have clamored for a special prosecutor or independent committee. They argue that congressional committees are too partisan to conduct credible probes. After Monday’s hearing, Trump took to Twitter to bash the media and deny any collusion. “Director Clapper reiterated what everybody, including the fake media already knows- there is ‘no evidence’ of collusion w/ Russia and Trump,” he said. And in another tweet, the president seemed to denounce the hearings. “The Russia-Trump collusion story is a total hoax, when will this taxpayer funded charade end?” he asked. FBI Director James Comey testified in the House on March 20 that the agency was investigating potential links between Trump associates and Moscow’s attempts to tilt the election. Trump had also used Twitter before the hearing to insinuate that Yates had leaked information on Flynn to the media. Yates and Clapper both swore under oath that they had never leaked classified information. Questioning on Monday often broke along party lines.     Some Republicans veered away from Russia to focus on issues such as whether the Obama administration had improperly revealed the names of Trump associates contained in surveillance records.  Senator John Cornyn, the No. 2 Republican in the Senate, was one of a handful who grilled Yates about her objections to Trump’s travel ban. Trump fired Yates after she defied the White House on the travel ban, a policy that Trump said would help protect Americans from Islamist militants. 