DSCC spends $250K in Indiana to attack Mourdock's uncompromising views

It follows a $600,000 ad from the GOP-aligned American Crossroads blasting Donnelly's record, a sign Republicans also are nervous about the race in the GOP-leaning state.

The ad, first obtained by The Hill, quotes Mourdock's comments after his primary win that "the highlight of politics, frankly, is to inflict my opinion on someone else," says he called Social Security "unconstitutional" and wants to cut Medicare while giving "more tax breaks to Wall Street."

The National Republican Senatorial Committee (NRSC) blasted Donnelly in their response to the ad.

"Hoosiers understand that Donnelly has been an ardent supporter of Speaker [Nancy] Pelosi and President Obama, including voting for ObamaCare, which cut Medicare by $500 billion and is causing health care costs to skyrocket across the country.  The hardworking men and women of Indiana simply cannot afford the Donnelly-Obama way of doing business," NRSC spokesman Lance Trover said in a statement.

The Hill rates this race a "toss-up."

— This story was updated at 1:24 p.m.