According to the poll, Donnelly has built up a big lead among independents, while Mourdock also has failed to consolidate the full support of the Republican base since his defeat of six-term Sen. Richard Lugar in the primary.
The survey, conducted Sunday through Tuesday by Democratic pollster Fred Yang and Republican Christine Matthews, includes 800 likely voters and has a margin for error of plus or minus 3.5 points.
“We have seen the utter collapse of Mourdock among the political 'middle,'” Yang wrote in an analysis of the poll. “Bottom line: Donnelly is likely to win next Tuesday, and probably the only 'suspense' will be the margin of his victory.”
According to Yang's analysis, Donnelly has a three-to-one advantage among independent voters, with 51 percent to Mourdock's 17 percent. And just 70 percent of Republican voters support Mourdock, even though 95 percent of GOP voters prefer Mitt Romney for president.
However, Mourdock appears to have relatively strong support in northeast Indiana, as he leads Donnelly 41 percent to 35 percent in the 3rd Congressional District. Of Indiana's nine Congressional districts, Mourdock leads in only the 3rd and 4th, the poll found.
The Howey/DePauw poll was the first independent poll to be released since the Oct. 23 debate where Mourdock tried to describe his view that when a rape victim becomes pregnant, the pregnancy is part of God's plan.
But Mourdock's campaign released its own poll showing the Republican ahead by 2 percentage points, making the race a statistical tie. Mourdock campaign officials argued that he had gained back much of the ground he had lost after the abortion comments.
“We certainly saw a little drop off in the first couple of days, and then we saw things stabilize and improve,” said Brose McVey, Mourdock's deputy campaign manager. “If you look at the average of polls that are out there, you get a credible sense that this is a tossup.”
Nine in 10 voters knew about Mourdock's abortion remarks, the poll found. Of those voters, 40 percent said the comments made them less likely to vote for Mourdock, while just 6 percent said it made them more likely to vote for him. The other 54 percent said the remarks made no difference in how they would vote.
Meanwhile, Republican-leaning pollster Scott Rassmussen released his own poll Friday that also showed Donnelly ahead 45 percent to 42 percent – within the margin for error.
“We think this race will be close, so we are working hard to ensure Joe is the next U.S. Senator from Indiana,” said Lizzi Shappell, a spokeswoman for Donnelly, in an email.
Howey/DePauw and Rassmussen are the only independent polls that have been active in Indiana this election cycle.
In the presidential election, Hoosiers favor Mitt Romney over President Obama by a 10-point margin, 51 percent to 41 percent, the Howey/DePauw poll found. For governor, Republican Mike Pence leads Democrat John Gregg 47 percent to 40 percent. State Superintendent of Public Instruction Tony Bennett, a Republican, leads Democratic challenger Glenda Ritz, a Carmel teacher, 40 percent to 36 percent, according to the poll.