Sen. Martha McSally (R-Ariz.) trails her Democratic opponent for the first time, according to a new survey, as President Trump's approval rating falls in the key swing state.

Retired astronaut Mark Kelly leads McSally, the retired fighter pilot-turned-politician, by a 46 percent to 41 percent margin, according to the poll conducted by OH Predictive Insights, a Phoenix-based pollster, and released first to The Hill.

The survey represents the first time this year that OH Predictive Insights has shown Kelly leading McSally. In February, the firm showed McSally leading by 2 percentage points. In May, McSally led by a single point.

McSally has fallen behind, the poll suggests, as President Trump's standing takes a hit in Arizona. Just 47 percent of likely voters in the state approve of Trump's job performance, down from 50 percent in May and 54 percent in October. His disapproval rating has risen from 46 percent last October to 52 percent today.

"Trump has seen a steady downward decline in his job approval," said Mike Noble, a Republican pollster and the managing partner and chief of research at OH Predictive Insights.

The poll shows a dangerous trend for McSally, who now trails Kelly by 9 percentage points in Maricopa County, home to about 3 in 5 Arizona voters. In the group's last poll, McSally trailed Kelly in Maricopa by 5 points.

In recent years, only one candidate, Diane Douglas, has won statewide election in Arizona without winning Maricopa County. Douglas won the election to be Arizona's superintendent of public instruction by just 16,000 votes in 2014, or about 1 percentage point; four years later, she didn't even make it out of the Republican primary, and a Democrat won Douglas's seat by 3 percentage points.

McSally lost her bid for a Senate seat in 2018 by 2.4 percentage points, or about 56,000 votes, to Sen. Kyrsten Sinema (D). Sinema carried Maricopa County by 61,000 votes. After that election, McSally was appointed to fill the state's other senate seat.

"Maricopa County is the problem," Noble said. McSally "had a problem in the previous poll where she was down five, and now she's down nine."

Kelly leads virtually every key demographic group. He runs 5 points ahead of McSally among men, by 6 points among women, by 2 points among white voters and by 18 points among Hispanics.

The poll shows few Arizonans are undecided about McSally; just 8 percent said they did not know enough about her to have formed an opinion. Among those who know her, 47 percent see her favorably, while 45 percent see her unfavorably.

Kelly, who is married to former Rep. Gabby Giffords (D-Ariz.), has more room to introduce himself. Forty-two percent of voters see Kelly favorably, while just 25 percent see him unfavorably. Almost 1 in 3 voters, 31 percent, have yet to form an opinion.

The race in Arizona, for a seat once held by the late Sen. John McCain (R), is likely to be one of the most costly and competitive of the cycle. Already, Kelly has raised more than $8.3 million, while McSally has pulled in $5.3 million, according to reports filed with the Federal Election Commission.

In 2018, the race between Sinema and McSally attracted $48 million in spending, between the two candidates and outside groups. Only five other contests attracted more spending that year.

The OH Predictive Insights survey polled 600 likely Arizona voters Aug. 13-14. The survey carried a margin of error of plus or minus 4 percentage points.