Former Sen. Claire McCaskill Claire Conner McCaskillDemocratic-linked group runs ads in Kansas GOP Senate primary Trump mocked for low attendance at rally Missouri county issues travel advisory for Lake of the Ozarks after Memorial Day parties MORE (D-Mo.) lambasted her former colleague Sen. Ron Johnson Ronald (Ron) Harold JohnsonSecond GOP senator to quarantine after exposure to coronavirus GOP-led panel to hear from former official who said Burisma was not a factor in US policy The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump contradicts CDC director on vaccine, masks MORE (R-Wis.) Tuesday morning on MSNBC's "Morning Joe," calling him an "embarrassing tool."

"Morning Joe" co-host Joe Scarborough Charles (Joe) Joseph ScarboroughScarborough calls on Cuomo to walk back statement he made about Trump: 'Out of bounds' Mika Brzezinski: 'Super grossed out' by Trump speech attendees 'who put their lives at risk' Democrats tear into Trump's 'deep state' tweet: His 'lies and recklessness' have 'killed people' MORE was talking about how President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE has continued to shake the hands of supporters, despite the recommendations from his coronavirus task force not to do so, when McCaskill steered the conversation to another topic: Johnson and his committee's investigation into Hunter Biden, the son of former Vice President Joe Biden Joe BidenCast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response Biden tells CNN town hall that he has benefited from white privilege MORE.

“Allow me to pop off on Ron Johnson here. What a joke! What an embarrassing tool!" she told Scarborough.

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"He is chairman of the Homeland Security Committee! Right? He has jurisdiction over coronavirus and the federal response. Is he calling a hearing? Is he trying to figure out why first responders’ tests aged on a shelf in the state of Washington until they were no longer good? Is he going to tell people why they can’t get the test? No," she continued.

"Even though it's not in his committee’s jurisdiction — the Judiciary Committee is not issuing subpoenas on Hunter Biden. The Foreign Affairs Committee is not issuing subpoenas on Hunter Biden. No! This joke, Ron Johnson, is actually politicizing a committee that right now should have one focus: What is going on with the United States of America’s federal response to the coronavirus? People should be really angry about this," McCaskill added.

On Monday, Johnson, chairman of Senate Homeland Security and Governmental Affairs Committee, signaled that he would move forward with a vote this week on a subpoena that is related to his investigation into Hunter Biden and the Ukrainian gas company Burisma Holdings. Senate Democrats have openly opposed the probe and subpoena, but it is expected that the measure to authorize the subpoena will pass.

"We have conducted our investigations methodically, responsibly, and largely out of public view," Johnson wrote to committee ranking member Gary Peters Gary Charles PetersBiden promises Democratic senators help in battleground states Postal service changes delayed 7 percent of nation's first-class mail: Democratic report GOP votes to authorize subpoenas, depositions in Obama-era probe MORE (D-Mich.) in a letter, which was released on Monday. "We have gone to great lengths to receive briefings and review and verify all information received by the Committee before making any of it public.

Peters has asked for the committee to receive a classified briefing regarding the subject of the subpoena from the FBI before voting on the subpoena. However, the voted is scheduled for Wednesday.