Clinton noted that President Bush has said he will veto the farm bill, which Congress passed Thursday. McCain, a senator from Arizona, also has said he would veto the bill if he were president.
"They're like two sides of the same coin, and it doesn't amount to much change, does it?" the New York senator said. "I believe saying no to the farm bill is saying no to rural America."
Bush and McCain both say the bill, which boosts farm subsidies and includes more money for food stamps, is fiscally irresponsible and too generous to wealthy corporate farmers.
"When Bear Stearns needed assistance, we stepped in with a $30 billion package. But when our farmers need help, all they get from Senator McCain and President Bush is a veto threat," Clinton said.
McCain, in a statement issued by his Senate office, said he recognized that "attempts will be made to use my opposition to this bill for another's political gain" but the "American people deserve to know the truth" about the bill and why he does not support it.
"It's a bloated piece of legislation that will do more harm than good for most farmers and consumers," he said.
Obama applauded the bill's passage in a statement released by his campaign, saying the measure was "far from perfect," but "with so much at stake, we cannot make the perfect the enemy of the good."
The Illinois senator also chided McCain and Bush for "saying no to America's farmers and ranchers, no to energy independence, no to the environment, and no to millions of hungry people."
Clinton chose South Dakota for her first campaign appearance since her West Virginia win earlier this week, signaling that she is sticking around until the final primaries on June 3 despite call from some Democrats to close ranks behind Obama. South Dakota and Montana vote that day - the finish line on the primary calendar.
"There are a lot of people who say, 'Well we should just wrap this up,"' Clinton told several hundred South Dakotans while standing on the porch of a fourth-generation family farmhouse in Bath. "Well I've never been impatient with democracy."
Meanwhile, in Kentucky, husband Bill Clinton urged voters to ignore those who say Obama will be the nominee. Kentucky holds its presidential primary on Tuesday.
"Your voice still counts," the former president said.
"They've tried to bury her more times than a cat has lives."
While chatting briefly with reporters as she flew to South Dakota, Clinton stuck to small talk, like describing the deer she occasionally sees outside her Washington home. She refused to comment on Edwards' endorsement. Both she and Obama had sought his backing.
Later, in Rapid City, she said she had a "great deal of respect" for Edwards.
"He and I have a lot in common ... I imagine that Senator Edwards' endorsement will be of some help to Senator Obama in Kentucky, but I think that what matters are the people who actually vote," Clinton said before heading to California for a fundraiser.
Clinton said she had not spoken with Edwards but had spoken with his wife, Elizabeth, about the endorsement. She declined to discuss their conversation.
The former first lady has maintained that her West Virginia triumph over Obama bolsters her argument that she would be the stronger nominee to face McCain in key states in the fall.
Left with an increasingly unrealistic mathematical path to the nomination, Clinton has turned to philosophical arguments in an attempt to appeal to the party leaders and elected officials, known as superdelegates, whose support will likely determine the nominee.
Suffering from money woes of more than $20 million in debt and trailing Obama in fundraising power, Clinton met with her finance team and top fundraisers at her Washington home on Wednesday to rally her forces. The message to the group was to remind them that she now has the lead in votes cast thus far throughout the primary season.
Her campaign continues to site a total, however, that includes results from the Florida and Michigan contests that the national Democratic Party has not recognized.
--- Associated Press writer Roger Alford in Louisville, Ky., contributed to this report.
(Copyright 2008 by The Associated Press. All Rights Reserved.) AP-NY-05-15-08 2049EDT
