Both parties raced to define the newly complete Republican presidential ticket Monday, as Mitt Romney’s selection of Wisconsin congressman Paul Ryan as his running mate began to reshape the race.

President Obama, beginning a bus tour in Iowa, sought to use Ryan’s seven terms in the House to lash the Republican ticket to dysfunction in Congress. In North Carolina, Vice President Biden deepened an assault on the GOP ticket over Ryan’s proposal to slash the federal budget and overhaul Medicare.

The Romney campaign tested Ryan’s ability to carry its message of a revived private sector, giving him the weighty task of going head to head against Obama in Iowa — a state the Democrat won four years ago.

And the Republican team gave a glimpse of how it hopes to deploy the 42-year-old: as an energetic charmer at ease campaigning in his native Midwest. On Monday, he took the spotlight in front of thousands at the Iowa State Fair in Des Moines. Far from playing the part of a conservative ideologue, as some had predicted, he avoided any mention of his signature effort to reform domestic entitlement programs. But he was heckled by protesters over his budget plans nonetheless, quickly transforming his first solo appearance as a national candidate into a chaotic spectacle.

With all four candidates on the two tickets hitting the trail simultaneously for the first time, new polls showed the necessity for both parties to quickly define Ryan. The numbers suggested that, despite his years on Capitol Hill, the congressman remains unknown to many Americans.

A new Washington Post-ABC News survey released Monday showed that positive views of Ryan increased by 15 percentage points after Romney named him to the ticket Saturday. But it also indicated that by Sunday, 30 percent of respondents still registered no opinion of the congressman.

A new USA Today-Gallup poll found that 39 percent of Americans think Ryan is an “excellent” or “pretty good” pick for a running mate — but 42 percent say he is a “fair” or “poor” choice.

Obama blamed the Republican Party, including Ryan, for inaction on an issue important to many Iowans: the renewal of the farm bill. Congress left for its August recess without approving new aid for farmers struggling amid a devastating drought.

“I am told that Governor Romney’s new running mate, Paul Ryan, might be around Iowa the next few days,” the president told thousands at Bayliss Park, a square in downtown Council Bluffs. “So if you happen to see Congressman Ryan, tell him how important this farm bill is in our rural communities. We’ve got to put politics aside and do the right thing for rural America and for Iowa.”

Obama campaign officials picked up the theme. Asked by reporters whether Ryan, whose House Budget Committee does not have jurisdiction over the farm measure, is to blame for its troubles, adviser Jen Psaki responded: “Well, Paul Ryan happens to be in Congress, as you may have heard. And he has not, as far as we can tell, taken steps to move the farm bill forward.”

Romney officials pointed out that Ryan supported the House’s disaster-relief bill. “The truth is, no one will work harder to defend farmers and ranchers than the Romney-Ryan ticket,” Romney spokesman Ryan Williams said.

Biden took an even more aggressive tone against his new rival. Campaigning in North Carolina, the vice president reminded voters of Ryan’s role in crafting proposals to dramatically reduce spending on education and infrastructure while lowering tax rates.

“What’s gutsy about gutting Medicare, Medicaid, education?” Biden said at a rally in Durham. “It’s not fair to the middle class and the working poor, and it will not grow the economy or reduce the deficit.”

The Romney campaign, still basking in a rush of attention two days after the Ryan announcement, offered less of a coordinated message than its Democratic counterparts. But with both Romney and Ryan in states with sizable elderly populations — the presumptive nominee spent the day in Florida — both men avoided focusing on Ryan’s plans for Medicare, the health-care program for retirees and those older than 65.

Generally, Republicans believe they can blunt Democratic attacks on Ryan’s Medicare proposal by going on the offensive: Emphasizing that his plan was a politically courageous attempt to salvage the program by halting its unmanageable growth.

But on Monday, Romney declined to offer a forceful defense of Ryan’s budget. And, after indicating in an interview Sunday on CBS’s “60 Minutes” that he would run on his own budget plans — not Ryan’s — Romney, appearing at a Miami news conference, repeatedly declined to outline what differentiates his ideas from Ryan’s.

“I’m sure there are places that my budget is different than his, but we’re on the same page,” he said. “As I’ve said before, we want to get America on track to a balanced budget. . . . My plan for Medicare is very similar to his plan for Medicare.”

Ryan appeared eager to emphasize other aspects of Romney’s economic message Monday, an effort muddied when hecklers disrupted his remarks.

About half a dozen protesters at the Iowa State Fair repeatedly interrupted the congressman, rushing the stage in a scene uncharacteristic for the normally highly scripted Romney campaign. “Are you going to cut Medicare?” one woman screamed from a few yards in front of Ryan.

“Stop the war on the middle class!” yelled an older, bespectacled man.

Ryan seemed to take the disruption in stride, continuing his stump speech without teleprompters or notes. “One thing we’ve got to get straight — one thing we got to get straight is we’re not growing this economy like we need to,” he said. “We’re not creating jobs like we can in America. And that is why Mitt Romney and I have a plan for a stronger middle class to get this country back on track, get this country growing jobs again and get us back on the path to prosperity in this country.”

Helderman reported from Washington. Philip Rucker in St. Augustine, Fla.; Felicia Sonmez and Ed O’Keefe in Des Moines; and Jon Cohen in Washington contributed to this report.