The race for North Dakota's open U.S. Senate seat continued to gain national prominence Tuesday as the Democratic Senatorial Campaign Committee released its second ad in the contest.

The Scrabble-themed ad, called "Spelled," criticizes Republican Senate nominee Rick Berg for failing to vote with North Dakota's needs in mind since becoming the state's sole congressman in 2011. The ad hits Berg for voting to end Medicare, give tax breaks to millionaires and raise his own pay. It will air statewide through July 2.

Berg is running against Democrat Heidi Heitkamp, a former state attorney general, for the seat being vacated by retiring Sen. Kent Conrad (D). The competitive race has become an increasingly important one in the battle for Senate control, with Heitkamp maintaining a one-point lead in a recent poll. Karl Rove's Crossroads GPS released an ad targeting Heitkamp last week.

The new ad is the second the DSCC has run in the race, following an earlier one that aired in April.

A Democratic source told HuffPost that the DSCC is spending $86,000 to run the commercial statewide for the next two weeks, a similar amount to what Rove's group spent against Heitkamp.

The Berg campaign could not immediately be reached for comment on the DSCC ad.

Heitkamp herself released a new spot this week, in which she dismisses negative ads, noting that she beat breast cancer 12 years ago and "when you live through that, political attack ads seem silly."