In the two years following the presidential election, Sanders continued his frequent private jet travel, spending at least $342,000 on the flights.

Increased scrutiny of his travel practices, which are at odds with his positions on wealth inequality and climate change, are among the challenges Sanders will face as he makes his second White House run.

But he also faces another hurdle: hard feelings that remain to this day after the contentious 2016 Democratic primary. Many in the party continue to believe the Vermont senator played a role in contributing to Clinton’s defeat in November because of his criticisms of her prior to the general election, and his refusal to concede earlier when it appeared he had little mathematical chance of securing the party nomination.