Sen. Mark UdallMark Emery UdallSenate GOP rejects Trump’s call to go big on gun legislation Democratic primary could upend bid for Colorado seat Picking 2018 candidates pits McConnell vs. GOP groups MORE (D-Colo.) is again hammering his Republican opponent, Rep. Cory GardnerCory Scott GardnerPoll: Almost two-thirds of Texas voters support legal recreational marijuana House, Senate GOP compete for cash Overnight Cybersecurity: Senators want info on 'stingray' surveillance in DC | Bills to secure energy infrastructure advance | GOP lawmaker offers cyber deterrence bill MORE, for supporting restrictions on abortions and birth control in a new ad.

It’s the latest back-and-forth over Gardner’s support of a so-called “personhood” measure, which would have given legal rights to a fertilized human egg, effectively outlawing abortion and certain forms of birth control. And in a sign of how damaging Udall’s focus on the issue could ultimately become, Gardner hit back in a web video of his own.

In Udall’s ad, the senator says that Gardner “led a crusade that would make birth control illegal, and sponsored a bill that would make abortion a felony.”

“His record is beyond troubling. It’s wrong. We’re talking about your rights, as women, as families, as Coloradans,” Udall says. 

Gardner has, since entering the race, shifted his position on a personhood initiative in Colorado and now says he's opposed, but he remains a co-sponsor of federal personhood legislation.

In his response video, Gardner admits that he changed his mind on the issue, but says he did so after listening to voters — something he says “Udall and President Obama can’t relate to.”

“Their takeover of health care is a disaster. We all know it. Yet Udall and Obama refuse to listen – while everyone else pays for it,” Gardner says in the ad.

The two are locked in a tight battle for the seat, which became more competitive with Republicans as issues with the ObamaCare rollout and Obama’s persistent unpopularity have contributed to a difficult climate for Democrats. Most polling of the race has shown a single-digit fight, with Udall slightly ahead.