CHESTER, Va. -- Barack Obama hammered at rival John McCain's personal wealth, part of the Democrat's more-focused message on the economy and the latest sign Sen. Obama is adopting attack politics to define his opponent.

Sen. McCain "said the economy is fundamentally strong," Sen. Obama told 250 people gathered outside John Tyler Community College. "If you don't know how many houses you have, then it's not surprising that you might think the economy was fundamentally strong."

When asked on Wednesday how many houses he owns, Sen. McCain told Politico, a Washington political news organization, that he wasn't sure. "I think -- I'll have my staff get to you," the Arizona senator said in Las Cruces, N.M. "It's condominiums where -- I'll have them get to you."

Sen. McCain and his wife, Cindy, an heiress to a beer distributor fortune of a reported $100 million, own several homes, including a trio of condominiums in Phoenix, Southern California and northern Virginia, in addition to a ranch in Sedona, Ariz., where Sen. McCain took a break from public events Thursday. The McCain campaign Thursday didn't respond to requests for details of the senator's properties. Politico's Web site reported that the senator's family owns at least eight properties.

The gaffe comes in the midst of a housing crisis and faltering economy that Sen. Obama has tried to seize on recently. He said Sen. McCain's "seven homes" show a "fundamental gap of understanding" of average Americans.

On Thursday, the Obama campaign dispatched its top surrogates to discuss Sen. McCain's statement and launched a new, 30-second television ad called "Seven." In the ad, the image of the White House appears and a man's voice says, "Here's one house America can't afford to let John McCain move into."

Republicans fired back, accusing Sen. Obama of receiving a preferential real-estate deal because of his ties to Chicago businessman Antoin "Tony" Rezko, a financial contributor to Sen. Obama's past political campaigns who was convicted in June on 16 counts of corruption.

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"As the McCain campaign desperately tries to find out how many houses John McCain owns, they are launching false and thoroughly debunked attacks," said Nick Shapiro, an Obama campaign spokesman. "Barack Obama offered the best price of any bidder in the market for his home -- the only home he owns -- every dime of which he paid for himself."

Neither presidential candidate can be seen as struggling economically. In a memo to reporters, the Republican Party said the Illinois senator owns a three-story home complete with four fireplaces and a wine cellar in Chicago's tony Kenwood neighborhood.

"Does a guy who made more than $4 million last year, just got back from vacation on a private beach in Hawaii and bought his own million-dollar mansion with the help of a convicted felon really want to get into a debate about houses?" McCain spokesman Brian Rogers said in a statement.

Sen. Obama's more-assertive approach comes as new polls show that Sen. McCain's recent barrage of negative ads portraying his opponent as a celebrity who isn't ready to lead may have helped the Republican candidate's presidential prospects. According to a Wall Street Journal/NBC News poll conducted this month, Sen. Obama leads among likely voters with 45%, to 42% for Sen. McCain, a statistical dead heat. A month ago, the presumptive Democratic nominee had a six-point advantage.

A Republican strategist said Sen. McCain's slip is "a great symbol for Obama to try to paint McCain as out of touch." But he said it will take some time to make that attack line stick.

Sen. Obama first showed this more-assertive side last weekend, when he came down hard on Sen. McCain for saying during an interview with an evangelical leader, the Rev. Rick Warren, that he would define "rich" as someone who makes $5 million or more.

--Elizabeth Holmes in Sedona, Ariz., contributed to this article.

Write to Amy Chozick at amy.chozick@wsj.com

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