Three U.S. Democratic senators have sued to block President Donald Trump's appointment of acting Attorney General Matthew Whitaker, alleging the appointment was made to undermine the ongoing criminal investigation of the 2016 Trump campaign's alleged links to Russia.Senators Richard Blumenthal of Connecticut, Sheldon Whitehouse of Rhode Island and Mazie Hirono of Hawaii filed the lawsuit Monday in a federal court in Washington.The suit is the fourth legal challenge of Trump's appointment of Whitaker, following the ouster earlier this month of Attorney General Jeff Sessions, whom Trump had long disparaged for removing himself from oversight of special counsel Robert Mueller's investigation.Before joining the Justice Department as Sessions's chief of staff more than a year ago, Whitaker attacked the Mueller investigation in commentary on television network CNN, saying a replacement attorney general, such as he is now, could cut funding to the probe so that it "grinds almost to a halt."Whitaker has taken no public action against the investigation since Trump named him, for up to 210 days, as the country's top law enforcement official, but also has made no statements on how he views the probe.Democratic lawmakers, along with some Republicans, have called for Whitaker to avow he would not curtail Mueller's investigation while it is still underway and contended his appointment, as head of a Cabinet-level agency, was subject to Senate confirmation. "President Trump is denying senators our constitutional obligation and opportunity to do our job: scrutinizing the nomination of our nation's top law enforcement official," Blumenthal said in a statement. "The reason is simple: Whitaker would never pass the advice and consent test.In selecting a so-called 'constitutional nobody' and thwarting every senator's constitutional duty, Trump leaves us no choice but to seek recourse through the courts."Senator Whitehouse added that the "stakes are too high to allow the president to install an unconfirmed lackey to lead the Department of Justice, a lackey whose stated purpose, apparently, is undermining a major investigation into the president.Unless the courts intercede."He added that this "troubling move creates a plain road map for persistent and deliberate evasion by the executive branch of the Senate's constitutionally mandated advice and consent. Indeed, this appointment appears planned to accomplish that goal."Justice pushes backThe Justice Department, for the second time in recent days, defended Whitaker's appointment as legal."There are over 160 instances in American history in which non-Senate confirmed persons performed, on a temporary basis, the duties of a Senate-confirmed position," a Justice Department spokeswoman said. "To suggest otherwise is to ignore centuries of practice and precedent."In an interview with Fox News that aired Sunday, Trump said he was unaware of Whitaker's CNN commentary opposing the Mueller investigation before naming him to head the Justice Department, bypassing Deputy Attorney General Rod Rosenstein, whom Sessions had delegated to oversee the Mueller investigation.Trump dismissed concerns about how Whitaker will deal with the Mueller investigation, but said that he, as president, would not intervene."It's going to be up to him," Trump said. "I think he's very well aware politically. I think he's astute politically. He's a very smart person. A very respected person. He's going to do what's right. I really believe he's going to do what's right."Asked by Fox News anchor Chris Wallace whether he would overrule Whitaker if he decides to curtail the Mueller investigation, Trump replied, "I would not get involved."Trump has answered written questions from Mueller about his campaign's connections with Russia during the run-up to the November 2016 voting, but told Wallace he "probably" won't sit for an in-person interview with Mueller's investigators.