NEW YORK, Sept 21 (Reuters) - The unemployment rate rose last month in some states considered key to the U.S. presidential election, including Iowa and Nevada, data from the Labor Department showed on Friday. Overall, the change in state unemployment rates across the country was a mixed bag. Jobless rates rose in 26 states in August from July, fell in 12 states and the District of Columbia, and was unchanged in another 12 states. Compared to last year, jobless rates fell in 42 states, rose in seven and remained the same in one. Because of the unique U.S. political system where states cast electoral votes for president, the contest between President Barack Obama and Republican candidate Mitt Romney is heating up in some states where polling suggests voters are undecided. The slow pace of economic growth and high level of unemployment has been a focal point of the campaigns ahead of the election in November. Of the eight states where the polls are currently tight, half had unemployment rates above the national average of 8.1 percent. Nevada, considered to be one of the battlegrounds of the election, had the highest unemployment rate in the country at 12.1 percent, up from 12 percent in July. The level of unemployment rose in other such swing states, with North Carolina climbing to 9.7 percent, and New Hampshire up at 5.7 percent. Florida, Ohio and Virginia all held steady, while Colorado eased to 8.2 percent from 8.3 percent.