Elections are run by the states including for federal office. They are not run by the federal government
Nor are there is no provision anywhere to cancel elections due to war. They still happened during the US Civil war
Hell even if in a hypothetical that it was just blue states that held elections, that would be enough to potentially flip the house. California and New York have enough potential and realistic house flips to change the US from red to blue
Declaring that they won't happen just plays into Trump's hands. It makes people more cynical and less inclined to fight back
Because they do have power if they're willing to use it. It can slow them down. For instance,
They have subpoenas. Make republicans have to spend all their time talking about all the horrible things they're doing to the floor. Remove all their time to do said horrible things because they're too busy testifying
If they don't testify, they can put people in contempt of congress which can be enforced by the Sergeant at Arms. This is something that did happen to Steve Bannon when he refuesed to testify for the Jan 6th committee. Not just a hypothetical power