[Publius]

I see three possibilities:

• A voting scandal becomes so great that the Democrats have no choice but to vote for it.
• Enough Democrat congressmen have to resign that the Republicans take the House for the rest of the year.
• The election is conducted during a period of national upheaval and is supervised by the military.

[MileHi]

•A voting scandal becomes so great that the Democrats have no choice but to vote for it.

Not sure where this scandal would come from before Nov.

•Enough Democrat congressmen have to resign that the Republicans take the House for the rest of the year.

Even if that happened, blue states would say they can't possibly make the changes before Nov. and some fed judge will stay the changes until the 2022 elections.

•The election is conducted during a period of national upheaval and is supervised by the military.

This could happen if POTUS maintains the state of emergency through the end of the year.

My $0.02

[aMorePerfectUnion]

Perhaps...

4. Enough Democrat congressmen are indicted...