Tommy Thompson's campaign said the Republican Senate candidate sold stock in four companies for $38,000 after learning they have ties to Iran.

Campaign spokeswoman Lisa Boothe said the companies included Royal Dutch Shell and Schlumberger.

Thompson first revealed the stock sale Thursday during a debate with U.S. Rep. Tammy Baldwin, his Democratic opponent. He said he doesn't tolerate doing business with Iran.

The sale represents a small fraction of Thompson's wealth. He told the Milwaukee Journal Sentinel he has at least $13 million in assets, plus more than $5 million in income since the start of 2010.

Thompson's campaign has criticized Baldwin for taking nearly $60,000 in donations from a group that claims Iran poses no threat to the U.S.

Recent polls show the race is a dead heat.