Washington (CNN) To win a presidential primary -- especially one with two dozen candidate -- you need some luck. And Joe Biden got some in this week's back-to-back debates in Detroit when several of his rivals repeatedly ran down the actions of President Barack Obama (and his administration) as a way at trying to lessen the former vice president's ability to claim credit.

Elizabeth Warren and Bernie Sanders openly advocated for the elimination of the Affordable Care Act in favor of a program that would abolish private insurance altogether. Julián Castro and Bill de Blasio bashed the rate of deportation of undocumented workers in the Obama administration. Jay Inslee took shots at the lack of major action taken during the previous eight years to address climate change.

And on and on it went. Candidate after candidate trying to drag Biden down by lessening the accomplishments of the President under whom he served.

Which, to my mind, is a huge political miscalculation.

Here's why: Obama left office as an immensely popular politician in the eyes of Democratic voters. In a Pew poll in July 2018 , more than 7 in 10 Democrats said that Obama was the best (51%) or second best (20%) president of their lifetimes. A Gallup poll in January 2017 showed 95%(!!) of Democrats approved of the job Obama had done in office.

Read More