WASHINGTON — Democratic Sen. Joe Donnelly has a slight advantage over Republican challenger Mike Braun in Indiana’s closely watched Senate contest, according to a new NBC News/Marist poll.

In a head-to-head race, the poll finds Donnelly with the support of 49 percent of likely voters, compared with 43 percent who back Braun. Among all registered voters, it’s a similar margin — 48 percent for Donnelly and 42 percent for Braun. (The margin of error for likely voters in the poll is +/- 5 percentage points.)

In a three-way contest that includes Libertarian Lucy Brenton, it’s 44 percent for Donnelly, 41 percent for Braun and 8 percent for Brenton among likely voters.

Donnelly, a low-key freshman senator in a state that voted for President Donald Trump by nearly 20 points, has long been considered one of the most vulnerable Democrats in the chamber. And Trump remains above water in the state with likely voters now, with the poll finding his approval at 48 percent approve, 46 percent disapprove. (Among all adults in Indiana, it’s roughly flipped: 44 percent approve, 47 percent disapprove.)

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But the poll also shows that Donnelly is being buoyed by relatively favorable personal ratings among Indiana voters. His favorability rating with likely voters stands at 48 percent positive, 31 percent negative. That includes a favorable rating of 79 percent among all Democrats who are likely voters, 50 percent of independents and 24 percent of Republicans.

Braun, who won a contentious primary against two Indiana congressmen by running as a political outsider, is also above water but is still somewhat unknown in the state. Among likely voters, 39 percent have a favorable impression of him, while 32 percent have an unfavorable impression and 29 percent have not heard enough about him to have an opinion. That includes a favorable rating from 71 percent of all Republicans, 30 percent of independents and just 8 percent of Democrats.

In a head-to-head contest, Donnelly is winning among independents (59 percent of likely voters compared with 30 percent for Braun), moderates (62 percent to 28 percent), whites with a college degree (51 percent to 45 percent), women (53 percent to 37 percent), and those who live in Indianapolis’ Marion County (71 percent to 24 percent).

Donnelly also captures 10 percent of Republican likely voters, compared with just 2 percent of Democrats who back Braun.

Some of Braun’s key supportive groups are Trump backers (79 percent compared with 15 percent for Donnelly), rural voters (52 percent to 38 percent), whites without a college degree (47 percent to 42 percent), white evangelicals (61 percent to 32 percent) and those who live in the Indianapolis suburbs (54 percent to 40 percent.)

Despite Trump’s net positive job approval rating among likely voters, more say they want their vote to send the message that more Democrats are needed in Congress to serve as a check on Trump. Forty-nine percent want more Democrats to be a check and balance, while 42 percent say they prefer more Republicans who will help Trump craft his agenda.

Jobs, health care are major issues in the Indiana Senate contest

Indiana registered voters name the economy and health care as the top issues that will decide their vote in November. A quarter — 26 percent — name the economy and jobs, 23 percent name health care, and an additional 14 percent cite immigration as their top issue.

Among Democrats, health care is the top issue by double digits, with 37 percent naming it as their No. 1 priority. Among Republicans, 28 percent say the economy and 21 percent cite immigration, with only 10 percent naming health care as the most important factor.

One policy of the president’s that is not particularly popular in the state is his move to raise tariffs and trade barriers on imports from abroad. Four-in-10 Indiana registered voters believe the tariffs will hurt the economy, while just 28 percent say they will help the economy. Only half of Republicans — 48 percent — say definitively that the moves will help the economy, with an additional 18 percent saying they will hurt and 24 percent saying they will have little impact.

The poll of 955 Indiana adults was conducted Aug. 26-29, 2018. The margin of error for all adults is +/- 3.9 percentage points. The margin of error for 816 registered voters is +/- 4.2 percentage points. The margin of error for 576 likely voters is +/- 5 percentage points.