Democratic incumbent Sen. Joe Donnelly holds slim lead over Trump-backed Mike Braun in Indiana: NBC News/Marist poll

  • Centrist Democrat Joe Donnelly holds a narrow edge in the Indiana Senate race over Mike Braun, a Republican businessman backed by President Donald Trump, according to a new NBC News/Marist poll.
  • The survey, released Wednesday, said 44 percent of likely voters support Donnelly, compared with 41 percent backing Braun in a three-way race. A Libertarian candidate, Lucy Brenton, gets the support of 8 percent.
  • Republicans see Indiana as one of their best opportunities to flip a Democratic seat as they try to keep or expand their 51-49 majority in the Senate. Trump carried the Hoosier State by roughly 20 percentage points in 2016.
Mike Braun and Sen. Joe Donnelly 
Tom Williams/CQ Roll Call)
Mike Braun and Sen. Joe Donnelly 

Centrist Democrat Joe Donnelly holds a narrow edge over Mike Braun, a Republican businessman backed by President Donald Trump, in the crucial Indiana Senate race, according to a new NBC News/Marist poll.

The survey, released Wednesday, said 44 percent of likely voters support Donnelly, compared with 41 percent backing Braun in a three-way race. A Libertarian candidate, Lucy Brenton, gets the support of 8 percent.

In a head-to-head contest, Donnelly leads Braun 49 percent to 43 percent, just outside the 5 percent margin of error for likely voters. Among registered voters, the Democrat has the support of 48 percent, compared with the Republican's 42 percent.

Republicans see Indiana as one of their best opportunities to flip a Democratic seat as they try to keep or expand their 51-49 majority in the Senate. Trump carried the Hoosier State by nearly 20 percentage points in 2016, while Donnelly won his seat in 2012 by about 6 percentage points.

Polling has been sparse, but Indiana is one of the most hotly contested Senate races this year. Trump and national Republican and Democratic groups have already gotten heavily involved. Taking into account the general election campaigns and outside groups, Indiana is the fifth-most expensive Senate race so far this year, according to the Center for Responsive Politics.

In his effort to keep his seat, the Democrat has tried to highlight bipartisan credentials. He has voted with Trump's priorities about 55 percent of the time, including on the president's immigration proposal, on a bill loosening some of the Dodd-Frank bank regulations and on conservative Justice Neil Gorsuch's nomination to the Supreme Court.

A Donnelly campaign ad released last week highlighted his vote "to build a wall and protect our borders," over video of him shaking the president's hand. It outlined his efforts to protect the health care of people in the state with pre-existing conditions and to put "Indiana ahead of party."

The message appears to be buoying Donnelly, according to the NBC News/Marist poll, which pegs Trump's approval rating in the state at 48 percent versus 46 percent disapproval. The senator leads his main challenger among independents (59 percent to 30 percent) and moderates (62 percent to 28 percent) in a two-way race. The Democrat also garners 10 percent of likely voters who identify as Republican.

Donnelly, like many Democrats hoping to win elections in red areas this year, has focused heavily on health care. In a tweet Tuesday, his campaign shared a story about Vice President Mike Pence's remarks that Republicans will try again to repeal the Affordable Care Act next year if they keep control of the chamber. Efforts to roll back the law and replace it failed in the Senate last year.

"Make no mistake: affordable, accessible health care is on the ballot in November," his campaign wrote in a tweet.

Registered voters peg health care as among the top two issues that will help them decide their vote. Twenty-six percent say the economy and jobs are their main concern, while 23 percent said health care.

One of Trump's key economic policies, his tariffs on billions of dollars in goods imported from allies and adversaries alike, didn't poll well with Indiana voters, according to the NBC News/Marist survey. Only 28 percent of registered voters said the tariffs will help the economy, while 40 percent said they would hurt it. Just under half of Republicans, 48 percent, said the tariffs would definitely help the economy.

Braun, a businessman and former state representative, has touted his record creating jobs in the state. He has also argued that Donnelly is too liberal for Indiana, repeatedly tying him to Senate Minority Leader Chuck Schumer, a New York Democrat. The president has already been to Indiana to stump for Braun.

A recent ad released by the Braun campaign features Trump calling Donnelly a puppet of Democratic leaders during a rally in Indiana last week.

"They say, 'Joe this is the way you're going to vote,'" Trump says of top Democrats.

Braun has recently hammered Donnelly over his lack of commitment so far on whether he will vote for Judge Brett Kavanaugh, the president's choice to succeed the retired Anthony Kennedy on the Supreme Court. Kavanaugh faced the Senate Judiciary Committee for his second straight day of confirmation hearings Wednesday, and the full chamber is expected to confirm him before the midterm elections despite staunch Democratic opposition.

The poll of 955 Indiana adults was conducted Aug. 26-29, 2018. The margin of error for all adults is +/- 3.9 percent. The margin of error for 816 registered voters is +/- 4.2 percent. The margin of error for 576 likely voters is +/- 5.0 percent.

— Graphic by CNBC's John Schoen.