The race for Democratic Sen. Claire McCaskill's coveted seat in Missouri is neck and neck, a new poll finds.

McCaskill leads Republican challenger Josh Hawley by 4 percentage points among likely voters when third party candidates are included, according to an NBC News/Marist poll released Tuesday. The senator garners 44 percent of support and Hawley draws 40 percent, while Libertarian Japheth Campbell and Green Party candidate Jo Crain get 5 percent and 3 percent, respectively. McCaskill's edge among likely voters falls within the poll's margin of error, signaling a tight contest in one of this year's most important Senate races.

In a separate question asking likely voters to decide between only McCaskill and Hawley, the candidates tie at 47 percent. Hawley, the state attorney general, has a 1 percentage point advantage among registered voters in a head-to-head matchup.

President Donald Trump may not help Hawley much in a state Trump won by nearly 20 percentage points in 2016. Forty-five percent of likely voters approve of the job the president is doing, versus 46 percent who disapprove, according to the survey.

In addition, the president's aggressive trade policy hasn't resonated with Missouri voters, according to the poll. Only 28 percent of likely voters believe tariffs — which Trump has imposed on major trading partners as he seeks new trade deals — protect jobs and help the economy. Meanwhile, 45 percent say the tariffs will raise the cost of goods and hurt the economy. Fourteen percent think the duties will not have much effect.

The survey results reaffirm why Republicans have made McCaskill one of their top targets this year. Defending a seat in a state Republican presidential candidates last lost in 1996, McCaskill is considered among the Senate incumbents most likely to lose in November.

Democrats hope to hold on to the seat as they try to stop the GOP from expanding its 51-49 seat majority in the Senate. They face a daunting task, as 26 Democrats and independents who caucus with them run for re-election this year. Only nine Republican Senate seats are on the ballot in November.

Money has already poured into Missouri. General election candidates and outside groups supporting them have spent $46 million on the race, second highest among Senate contests, according to the Center for Responsive Politics. The pro-Democratic Senate Majority PAC and the National Republican Senatorial Committee have led the way in outside spending.