The organization's "Pending Home Sales Index" fell 5.6% to 101.6 in September from a downwardly revised 107.6 in August, and is 1.2% below September 2012 when it was 102.8. The index is at the lowest level since last December when it was 101.3.
Lawrence Yun, NAR's chief economist, said concerns over the government shutdown also played a role.
"Declining housing affordability conditions are likely responsible for the bulk of reduced contract activity," he said. "In addition, government and contract workers were on the sidelines with growing insecurity over lawmakers' inability to agree on a budget."
"A broader hit on consumer confidence from general uncertainty also curbs major expenditures such as home purchases," he added.
Yun noted that September marked the first time in 29 months that pending home sales weren't above year-ago levels.
"This tells us to expect lower home sales for the fourth quarter, with a flat trend going into 2014," Yun said. "Even so, ongoing inventory shortages will continue to lift home prices, though at a slower single-digit growth rate next year."
NAR's Pending Home Sales Index in the Northeast dropped 9.6% to 76.7 in September, and is 6.4% below a year ago. In the Midwest the index fell 8.3% to 102.3 in September, but is 5.7% higher than September 2012. Pending home sales in the South slipped 0.4% to an index of 116.2 in September, but are 2.0% above a year ago.
The index in the West dropped 9.0% in September to 97.3, and is 9.8% lower than September 2012.
Total existing-home sales this year will be 10% higher than 2012, reaching more than 5.1 million, and are likely to hold up well even in 2014. The national median existing-home price is expected to rise 11 to 11.5% for all of 2013, but moderate to a 5 to 6% gain in 2014.