Democrat Kyrsten Sinema and Republican Martha McSally are locked in a tight race to succeed GOP Sen. Jeff Flake in Arizona: NBC News/Marist poll

Key Points
  • Democratic Rep. Kyrsten Sinema and Republican Rep. Martha McSally are locked in a close race to replace GOP Sen. Jeff Flake in Arizona, according to a new NBC/Marist poll. 
  • Sinema has a slight lead among likely voters in one of the year's most important Senate races. 
  • Republicans are trying to keep or expand their 51-49 seat majority in the chamber. 
Senate candidate Kyrsten Sinema speaks with volunteers in Phoenix in August. 
Matt York | AP

Democratic Rep. Kyrsten Sinema holds a slight edge in her bid to flip a U.S. Senate seat in Arizona in a contest with Republican Rep. Martha McSally, according to an NBC News/Marist poll released Tuesday.

In a race forecasters consider one of Democrats' two best chances to take a GOP-held seat, Sinema garners 48 percent of support from likely voters, versus 45 percent for McSally, the survey found. Seven percent of likely voters are undecided. The Democrat's edge falls within the 4.7-point margin of error.

When Green Party candidate Angela Green is included, Sinema's edge dips to 2 percentage points among likely voters. Green gets the support of 6 percent, while another 6 percent are undecided. Nine percent responded that their preference might change on Election Day.

Martha McSally, Republican U.S. Senate candidate from Arizona, speaks during an election night rally in Tempe, Arizona, on Tuesday, Aug. 27, 2018.
Caitlin O'Hara | Bloomberg | Getty Images

The representatives aim to replace Sen. Jeff Flake, a Republican and occasional critic of President Donald Trump who will retire in January. Keeping the senator's seat would give a big boost to the GOP as it tries to keep or expand its 51-49 seat majority in the chamber.

McSally's standing in the race against Sinema has improved since June, when the Republican was still locked in a bitter three-way primary. The Democrat had an 11-percentage point advantage then among registered voters, versus only a 3-point edge now.

Still, most recent public polls have found a lead for Sinema. Early last month, her campaign also had a slight lead in the money race. She had about $2.5 million in the bank, versus about $1.9 million for McSally's campaign, according to Federal Election Commission filings.

Arizonans have better views of Sinema than of McSally. Among likely voters, 46 percent view her favorably, while 33 percent see her unfavorably. Forty percent have a positive view of McSally, versus 42 percent who see her unfavorably.

The president could pose a challenge for McSally, like many other Republican candidates running in swing races this year. She tied herself more closely than before to Trump during the Republican primary in order to win over the state's GOP base. The representative now has to adjust to a general election campaign.

Forty-four percent of Arizona's likely voters approve of the job Trump is doing, while 51 percent disapprove, the survey found. But only 30 percent say they strongly approve, versus 45 percent who responded that they strongly disapprove.

These three factors will determine if Democrats will take back Congress
Three factors will determine if Democrats will take back Congress

In the overall battle for the legislative branch, 47 percent of likely voters said they would rather see a Democratic-controlled Congress after November's elections, while 44 percent responded that they would rather see the GOP keep majorities.

More than half of likely voters, 52 percent, say they want more Democrats in Congress to serve as a check on Trump. Only 38 percent want to see more Republicans to help the president pass his agenda.

The poll of 950 Arizona adults was conducted Sept. 16-20, 2018. The margin of error for all adults was 3.7 percentage points, for 763 registered voters 4.2 points and for 564 likely voters 4.7 points.

Democrats hope to flip 23 GOP-held House seats to take a House majority after November. Only a couple House races in Arizona are expected to be competitive this year, including McSally's 2nd District.

Nonpartisan election sites Cook Political Report and Sabato's Crystal Ball consider it a race that leans Democratic.

— Graphic by CNBC's John Schoen