At the present bitcoin doesn't have any defense against a single new miner with 51% overtaking the network.
That isn't necessary.

How could bitcoin defend against itself?

Every miner should sign the blocks that he solves with his own key. After a miner has created a lot of blocks
over month that get accepted by other miners, new blocks of the same miner deserve more trust than blocks
of new miners.

How do you defend against one established miner using this new voting power to attack?
Allow every client to make the choice to distrust a miner. If a miner attacks the network than the community
can decide to distrust the key of that miner.