As a corollary to that, he's also been outspending Clinton.
In practical terms, that means out-advertising her.
Last year, Sanders fans (and the campaign) noted that he'd received less media coverage than Clinton. Mediaquant, a company that tracks the value of the media attention that brands and candidates get across media platforms, now gives Sanders a score of 96 -- compared to Clinton's 97 and Donald Trump's 99. "Sanders is becoming somewhat of a media personality," a recent blog post from the company reported. The value of his media attention in March was $159 million -- and over the course of the campaign, Mediaquant estimates that he's gotten over $480 million in earned media. In February, the company stated that "Sanders has been able to beat his prior media performance with each successive month, and he’s done that better than Clinton over the last three media cycles."
Of course, Clinton's efforts have been aided by super PACs supporting her candidacy. Through the end of March, Washington Post analysis indicates that outside groups backing Clinton had spent about $73 million on her behalf -- a little less than Clinton's campaign has spent in 2016.
But, again, it's important to remember how far Sanders has come. His once long-shot campaign has outperformed nearly every other 2016 campaign, and is outperforming Clinton's on key metrics right now.
With one critical exception, of course: Delegates.