WASHINGTON  Senator Barack Obama is starring in a growing number of campaign commercials, but the latest batch is being underwritten by Republicans.

In a sign that the racial, class and values issues simmering in the presidential campaign could spread into the larger political arena, Republican groups are turning recent bumps in Mr. Obama’s road  notably his comment that small-town Americans “cling” to guns and religion out of bitterness and a fiery speech by his former minister in which he condemned the United States  into attacks against Democrats down the ticket.

“The public, week by week, is becoming more familiar with his big-government, far-left vision for America,” said Ed Patru, a spokesman for Freedom’s Watch, an advocacy organization that is portraying Mr. Obama as ultraliberal in an advertisement running in Louisiana before a special election for a House seat.

Republicans say the new focus on Mr. Obama reflects their view that he remains the more likely Democratic presidential nominee since he continues to lead Senator Hillary Rodham Clinton in convention delegates. It also shows that Republicans, who have for months characterized Mrs. Clinton as the contender who would most energize Republican voters, now see vulnerabilities in Mr. Obama that could be liabilities for other Democrats on the ballot.