CEDAR RAPIDS, Iowa -- Republican John McCain yesterday called for replacing the head of the Securities and Exchange Commission, and Barack Obama sharply condemned his presidential opponent for trying to run from a political record that the Democrat insisted has worsened the current economic crisis.

As the stock market continued this week's gyrations by posting a sizable gain after huge losses, the candidates used some of their toughest language yet as they tried to shape the campaign's leading issue.

Speaking at a rally in an airplane hangar in Cedar Rapids, Mr. McCain said the SEC, the primary regulator of Wall Street, had let "speculators and hedge funds turn our markets into a casino.

"The chairman of the SEC serves at the appointment of the president and, in my view, has betrayed the public trust," Mr. McCain said. "If I were president today, I would fire him."

Christopher Cox was appointed SEC chairman by President Bush in 2005. Before that, Mr. Cox was a Republican congressman from California for 17 years.

"While I have great respect for Senator McCain, we have sometimes disagreed, and this is one such occasion," Mr. Cox said in a statement that outlined steps the agency has taken to deal with the current difficulties. "History will judge the quality of our response to this economic crisis, but now is not the time for those of us in the trenches to be distracted by the ebb and flow of the current election campaign."

In an appearance in Espanola, N.M., Mr. Obama argued that Mr. McCain's call to replace Mr. Cox -- who said he had planned to leave when a new administration takes office in January -- was too little to deal with the magnitude of the economic problems that he said were tied to the Republican drive for government deregulation.

"Now that this disaster has hit, John McCain is calling for the firing of the SEC commissioner. Well, here's what I say," Mr. Obama told the cheering crowd. "In 47 days, you can fire the whole trickle-down, on-your-own, look-the-other-way crowd in Washington who have led us down this disastrous path."

Since the SEC was established as a nonpartisan agency, the president, by statute, cannot fire any of the SEC commissioners, including the chairman, although he appears to have authority to remove him from the chairman's seat for cause. The McCain campaign said their candidate was speaking colloquially.

Throughout the week, Democrats have attacked Mr. McCain for calling for a commission to study the current financial turmoil. Democrats have been especially critical of how Mr. McCain first opposed, then supported, an $85 billion bailout of American International Group, a major insurance company. Mr. McCain argued that the size and impact of a possible AIG bankruptcy forced him to reconsider.

Yesterday, Mr. McCain fired back. "Senator Obama talks a tough game on the financial markets, but the facts tell a different story," the Republican said. "He didn't lift a hand to avert this crisis."

Mr. McCain accused the Obama campaign of taking political advantage of the financial crisis. He also blamed the Illinois senator for his ties to former Freddie Mac and Fannie Mae executives -- including one who helped Mr. Obama find a vice-presidential candidate. Mr. McCain was referring to James A. Johnson, who headed the vice-presidential search efforts of both Mr. Obama and of Massachusetts Sen. John F. Kerry in 2004. Mr. Johnson was Fannie Mae chairman and chief executive from 1991 to 1998.In New Mexico, Mr. Obama attacked Mr. McCain for changing his position on the economy and the AIG rescue. "You can't just run away from your long-held views or your life-long record," the Democrat said. "You can't erase 26 years of support for the very policies and people who helped bring on this disaster with one week of rants."

In Iowa, Mr. McCain called for the creation of a federal agency that he said would work with regulators and the private sector to intervene with troubled institutions. The proposed agency, which Mr. McCain called the mortgage and financial institutions trust, or MFI, would "work with the private sector and regulators to identify institutions that are weak and take remedies to strengthen them before they become insolvent," he said.

The GOP rally outside Cedar Rapids was interrupted several times by protesters, who were shouted down by McCain supporters chanting "USA" before being escorted from the hangar.