The Democratic presidential candidates will face off on the debate stage again Thursday night, but the reality is that Elizabeth Warren is the only candidate who has gained any real momentum since the first debate, based on polls during the last three months.

The big picture: Post-debate polling spikes have proven to be sugar rushes. After jumping from 7% to 15% after the first debate, Kamala Harris is back down to 7%, while Julián Castro's cameo above 1% after that debate was also short-lived. Warren's ascent from 9% to 19%, meanwhile, has been gradual and steady.