Story highlights Sanders built on his new line of attack against Clinton on Friday night, playing off President Barack Obama's decision just hours earlier to kill the Keystone XL pipeline

Even O'Malley got in on the hits, saying Clinton got around to opposing the pipeline "just last week"

Rock Hill, South Carolina (CNN) The three Democrats left in the field may not have shown up onstage simultaneously Friday night here, but the two challengers running behind front-runner Hillary Clinton spent the night trying to hit her head-on.

Clinton, Vermont Sen. Bernie Sanders and former Maryland Gov. Martin O'Malley sat down, one after the other, with MSNBC's Rachel Maddow for a half hour at Winthrop University. And despite never facing off directly onstage, the hits sounded sharper than they had before, even during the first Democratic debate hosted last month.

Sanders built on his new line of attack against Clinton on Friday night, playing off President Barack Obama's decision just hours earlier to kill the Keystone XL pipeline.

"Now to me, as opposed to maybe some other unnamed candidates, the issue of Keystone was kind of a no-brainer. It never made sense to me, from day one, as to why you would extract and transport some of the dirtiest fuel on this planet," Sanders said, taking an implicit jab at Clinton, who consistently refused to say where she stood on the issue until recently.

Even O'Malley got in on the hits, saying Clinton got around to opposing the pipeline "just last week" (it was actually several weeks ago) while he, too, had been opposed for months.

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