The lawsuit, filed in Cole County Circuit Court, claims Nixon maliciously or recklessly broadcast false statements by describing Spence as a banker who used federal bailout money to buy a vacation home. The suit seeks unspecified monetary damages.
"I filed this lawsuit for one simple reason: We need good people in politics," Spence said in a prepared statement. "If the standard for truthfulness in political campaigns is there is no standard, our state and country will continue to erode because good people won't run for office."
Nixon campaign manager Oren Shur defended the ad and said TV stations across the state had rejected Spence's request that they stop running it.
"You see a lot of crazy stunts during the course of a campaign, but this frivolous lawsuit is misguided and desperate," Shur said.
Spence joined the board of Reliance Bancshares Inc. in May 2009, after the bank already had received $40 million from the U.S. Treasury under the Troubled Asset Relief Program, known as TARP.
The next year, Spence took out a more than $1.1 million mortgage on a vacation home at the Lake of the Ozarks. But Spence has said there was no connection between the federal money and the loan, he received no preferential treatment and he left the room when other board officials considered and approved the loan.
Among its chief claims, Spence's lawsuit says Nixon's ad is false for asserting he ran the bank and used bailout money to help himself.