In December 2013, with President Barack Obama's popularity sagging and a struggling economy, many quality Democratic candidates decided not to run in the 2014 elections. This demonstrates

The Strategic politician hypothesis

Explanation:

Strategic politician hypothesis involves politicians knowing when to make certain moves concerning their political ambitions.

President Barack Obama's popularity sagging and a struggling economy, many quality Democratic candidates decided not to run in the 2014 elections because they felt it wasn’t the right time to do so in order to support their fellow Democrats in order not to lose power to the opposition.