The unemployment rate stood below the 5% line in 15 states last month, the latest sign of a healthier labor market across a broad swath of the country.

The tally, which matched the largest number of states below that mark in six years, also highlighted how some regions, particularly the South and far West, are still struggling with elevated joblessness.

With the midterm elections just weeks away, the speed of the labor market's recovery could play an important role in at least a half-dozen states with close races for governor.

Oil-rich North Dakota had the nation's lowest unemployment rate at 2.8%, while Georgia had the highest reading at 7.8%, the Labor Department said Tuesday. The rate in Colorado fell to 4.7% from 5.1% in August, making it the latest state to slip below 5%.

Colorado Gov. John Hickenlooper, a Democrat, was quick to tout the latest figures. "Now is the time to keep at it, to seize momentum," he said Tuesday, while noting the state has added jobs for 35 straight months.

Mr. Hickenlooper is locked in a close race with Republican candidate Bob Beauprez, who noted earlier this month that while energy production has helped much of the state, "far too many Colorado families are still waiting for recovery to come to their kitchen table."

The jobless rate also fell in Florida, to 6.1%, the lowest reading since Gov. Rick Scott, a Republican, was elected in 2010. The rate was 11% when his opponent, former governor Charlie Crist, left office.

The national unemployment rate was 5.9% in September, and has fallen in most states over the past year. But not all incumbents can tout an improving economy.

The jobless rate fell in Georgia but is virtually unchanged from a year earlier. Georgia state senator and Democratic gubernatorial candidate Jason Carter has made the state's high jobless rate a focus of his campaign challenging incumbent Republican Gov. Nathan Deal.

The states with unemployment rates below 5% are concentrated in the middle of the country--and are generally more lightly populated, representing less than 12% of the country's population. There are fewer workers in those 15 states than in California.

Despite the improvement across the past year in most states, only four--North Dakota, Minnesota, Ohio and Michigan--had lower unemployment rates last month than in September 2007, a few months before the recession began.

North Dakota has benefited from jobs tied to oil and gas production, while Ohio and Michigan recorded increasing unemployment rates during 2007 as the manufacturing sector began to struggle. The rate last month in Ohio was 5.6%, while the rate in Michigan was 7.2%.

The jobless rate in Minnesota was 4.1% in September, down a half-percentage point from seven years earlier. The number reflects the state's diverse employment base, growth in health-care jobs at the Mayo Clinic and other providers and its proximity to North Dakota, said Laura Kalambokidis, economist at the University of Minnesota.

"The agriculture and oil boom is drawing workers over the border" to North Dakota, she said. And Minnesota was "not overly represented by sectors hurt hard by the recession, such as construction."

Write to Eric Morath at eric.morath@wsj.com

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