Donnelly, a first-term senator, faces one of the toughest re-election bids of anyone in the Senate this year as Republicans try to maintain or boost their 51-49 majority in the chamber. He attempts to hold a seat in a state President Donald Trump won by about 20 percentage points in 2016. The president has repeatedly visited the state to bash Donnelly and boost Braun, a former state representative and businessman.

When Libertarian Lucy Brenton is included, Donnelly's edge expands slightly to three percentage points. The survey has a margin of error of plus-or-minus 5.5 percentage points for the likely voter sample.

The incumbent leads his GOP challenger by two percentage points among likely voters, down from six percentage points in September, according to the NBC News/Marist survey that comes six days before Tuesday's midterms. Donnelly garners 48 percent of support, versus 46 percent for Braun, with 5 percent undecided.

Democratic Sen. Joe Donnelly is locked in a tight race with Republican Mike Braun as he defends his important seat in Indiana, a new poll released Wednesday found.

The Democrat has walked a tightrope in deep red territory this year, pledging to protect health care coverage for people with pre-existing conditions and highlighting his cooperation with Trump on issues such as immigration. But he has also made some nods to the Democratic base, voting against Supreme Court Justice Brett Kavanaugh's confirmation and scheduling a rally with former President Barack Obama this weekend.

Braun has cast himself as a Trump ally, hitting Donnelly for his votes against last year's GOP tax law and the Supreme Court justice. He has cast himself as a job creator who will boost the state's economy. But Braun, like many Republicans around the country, has had to play defense over pre-existing conditions coverage amid Democratic attacks.

Trump heads to Indianapolis on Friday, followed by an additional rally in Fort Wayne on Monday to try to leverage his appeal to GOP voters. Among Indiana likely voters, 50 percent approve of the job Trump is doing, versus 42 percent who disapprove, an improvement since September, according to the NBC/Marist survey. It includes 35 percent who strongly approve and 33 percent who strongly disapprove.

Voters appear to have better views of Donnelly than Braun, which could help him to overcome the state's red, pro-Trump leanings. Forty-six percent have a favorable view of the Democratic senator, while 38 percent see him unfavorably, the survey found. That compares with an even 41 percent who view Braun favorably and unfavorably.

Donnelly's opposition to Kavanaugh also may have turned some voters away from him, after he supported Justice Neil Gorsuch's confirmation last year. Forty percent of likely voters said they are more likely to vote for a candidate who supported Kavanaugh, versus 33 percent who responded that they prefer a candidate who voted against the justice.

The live-caller NBC/Marist poll of Indiana was conducted from October 24-28 among 931 adults (which has a margin of error of plus-minus 3.9 percentage points), among 800 registered voters (plus-minus 4.2 percentage points) and 496 likely voters (plus-minus 5.5 percentage points).

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