DES MOINES, Iowa (Reuters) - Republican Paul Ryan got a taste of the rough side of a presidential campaign on Monday when protesters heckled him and President Barack Obama accused him of blocking emergency aid to drought-hit farmers.

The new vice presidential hopeful from Wisconsin - who brings Midwestern credibility to White House hopeful Mitt Romney's campaign - mingled with locals at the Iowa State Fair, a popular spot for politicians keen to show their common touch in a state where Obama and Romney will be in tight competition.

Dressed in a checked shirt, Ryan bantered with fairgoers but his first solo campaign speech was interrupted by a small group of demonstrators.

In chaotic scenes, hecklers standing at the front of a large pro-Ryan crowd shouted "Stop the war against the common good," prompting Republicans to shout back and swear at them.

One of the protesters rushed onto the small stage where Ryan was speaking. She was grabbed by three state troopers. Police said one of the protesters punched a volunteer fair worker.

"It's funny because Iowans and Wisconsinites, we like to be respectful of one another, and peaceful of one another and listen to one another," Ryan said. "These ladies must not be from Iowa or Wisconsin."

A hero to conservatives, Ryan has given Romney's campaign a jolt of energy after several difficult weeks marked by gaffes and persistent questions about his personal finances.

But the Republican effort to win back the White House received no immediate poll boost from Saturday's announcement of Ryan as the vice presidential running mate, according to a Reuters/Ipsos online survey.

Some 51 percent of those surveyed said the decision did not change their opinion of Romney, a former private-equity executive and Massachusetts governor who faces Obama in the November 6 election.

Another 26 percent said they viewed Romney more favorably after he added the 42-year-old Wisconsin congressman to the ticket, while 23 percent said they regarded him less favorably.

STILL UNKNOWN

Ryan is a polarizing figure in Washington, where he has led his party's push to cut domestic spending, lower taxes and scale back the size of the federal government as chairman of the House of Representatives Budget Committee.

But Americans outside Washington and Ryan's home state know little about him.

"He's fairly unknown in who he is and what he stands for," said Ipsos Vice President Julia Clark. "He's a Wisconsin congressman, not a nationally known figure."

That is likely to change as Ryan campaigns across the country to build enthusiasm among grass-roots conservatives while Democrats attack his budget plan as one that would gut social safety net programs for the elderly and the poor.

Also visiting Iowa on Monday, Obama named Ryan as "one of the leaders of Congress standing in the way" of a farm bill that includes disaster aid to farmers hit by a drought.

"So if you happen to see Congressman Ryan, tell him how important this farm bill is to Iowa and our rural communities," Obama said at a political rally in Council Bluffs.

Later in the day the president visited the State Fair, too, but did not face hecklers the way Ryan did. Obama shook hands with cheering bystanders and turned down a smoothie and cinnamon rolls in favor of a beer and pork chops.

"Ice cold and tasty," he said of his beer, offering to buy some for others in the crowd except, he joked, a person holding a Romney sign.

During his Iowa tour Obama announced an emergency purchase of up to $170 million of meat and fish to help farmers who are watching crops wither in the fields under searing heat.

It was a timely announcement that took away some of the limelight in Iowa from Ryan, who has never before had to face an opponent like Obama's Chicago-based campaign team.

Elected to the House of Representative seven times mostly by margins of more than 20 percentage points, Ryan has not faced a particularly tough race in his congressional career.

He will go head-to-head with Vice President Joe Biden in October in a TV debate.

Biden criticized his Republican counterpart for targeting Medicare spending in his budget plan.

"What's gutsy about giving millionaires and billionaires tax breaks? What's gutsy about gutting Medicaid and Medicare?" Biden said in Durham, North Carolina.

Ryan argues that Medicare in its current form is headed for bankruptcy and his proposal would save the program by reducing spending by $205 billion in the next decade compared with Obama's budget plan.

Polls show that Medicare is very popular and Romney has distanced himself from Ryan's plan.

"The items that we agree on I think outweigh any differences there may be," Romney said on a visit to Florida. "We haven't gone through piece by piece and said, 'Oh, here's a place where there's a difference.' I can't imagine any two people, even in the same party, who have exactly the same positions on all issues."