Hillary Clinton remains in a dominant position in the Democratic presidential primary, but it's looking increasingly like Bernie Sanders is emerging as her main challenger — which means that Martin O'Malley isn't. Indeed, the former Maryland governor is barely topping 1 percent of the vote in national and early state polls.

As a result, the people behind the main pro-O'Malley Super PAC , Generation Forward, seem to be getting restless — so they've released a new web video attacking Sanders's record on guns, along with one praising O'Malley's.

But just hours later, Sanders gave what looked like an indirect response to the new video, on Twitter:

As someone who has never run a negative political ad in his life, my campaign will be driven by issues and serious debate. — Bernie Sanders (@BernieSanders) June 25, 2015

It's a rather remarkable testament to Sanders's success so far that what may be the first explicit attack video of the Democratic contest targets him — it makes it even clearer that Sanders has emerged as the clear second-place contender (though he's still well behind Hillary Clinton).

It is accurate that Sanders is pretty far from being a staunch liberal on the gun issue — likely reflecting the views of his rural Vermont constituents. However, in 2013, Sanders voted for the Democrats' post-Newtown gun control bill, which didn't become law, but would have expanded background checks and restored the assault weapons ban. He said then that there was "a growing consensus" that "we have got to do as much as we can to end the cold-blooded mass murders of innocent people."

Overall, though, the bigger problem for O'Malley is that activists on the left are excited about Sanders's candidacy in a way they just aren't about his own. And it doesn't seem likely that attack ads will solve that problem for him.