What do they want to investigate? He has been investigated already multiple times. There isn’t anything else to investigate. She is the one that should be investigated.
Trump and the Republicans called their bluff by offering to have her testify. This is simply a delay tactic by the dems. They believe the can win a majority in the mid term elections and if they can postpone a vote until they have a majority they can stop all of Trump's appointments.
If she doesn't show up Monday they should immediately hold a vote.
