Titled "I'm With Him," Clinton's straight-to-camera ad continues her campaign's knock against Sanders' more conservative positions on guns.
"An average of 90 people are killed in this country every single day. It has to stop," Clinton says. "President Obama wants to make universal background checks the law of the land and he wants to make gun manufactures can finally be held accountable when their guns are used to kill our children."
"It is time to pick a side," Clinton says in a not-so-subtle dig at Sanders. "Either we stand with the gun lobby or we join the President and stand up to them. I am with him. Please join us."
The ad will run online around Obama's final state of the Union and will start to run on national cable in Iowa and New Hampshire on Wednesday, according to aides. The online ad will include a petition to support Obama and Clinton's call for gun reform.
The ad caps off a difficult day for the Clinton campaign, where a trio of national and early state polls found numbers headed in Sanders direction.
In response, Clinton honed in on Sanders on the stump, drawing sharp contrast with Sanders on guns, healthcare and Wall Street.
Clinton hit Sanders at an event in Ames -- where she also picked up an endorsement from the Brady Campaign to Prevent Gun Violence -- for bragging about standing up to special interests, but siding with the National Rifle Association on certain legislation.
If Sanders is to tout his willingness to stand up to special interests, Clinton said at her events that Sanders should "stand up to the most powerful special interest, stand up to that gun lobby."
Sanders' campaign aides have said they support Obama's gun plan, but when the senator was asked whether he regrets his 2005 vote to give immunity to gun manufacturers, Sanders said "no" on Monday in Iowa.