When President Obama was first elected, aides say, he saw Rep. Paul Ryan, another ambitious Midwestern policy wonk, as someone he could possibly work with to reverse the building federal debt.

He soon would change his view, as Obama made plain Sunday in welcoming Ryan to the race as Mitt Romney's running mate. "The ideological leader of Republicans in Congress," Obama called him, affixing a label clearly not meant as praise and also saying Ryan was an advocate of the same "top down" economics as Romney.

"I know him, I welcome him to the race," Obama said at a Chicago fundraiser. "He is a decent man. He is a family man. He is an articulate spokesman for Gov. Romney's vision. But it is a vision that I fundamentally disagree with."

For his part, Ryan does not think much of the president's ideas either.

"He has put all of his policies in place, and they're just not working," he told a partisan crowd in Manassas, Va., Saturday evening. "Take a look at the results. We've got the worst recovery in 70 years."

The president and Ryan know each other well. For nearly four years the two, who will both campaign in Iowa on Monday, have been the leading antagonists in an increasingly rancorous debate over the size and scope of government — a debate forced by projections of unsustainable, mounting debt as the population ages and health-care costs keep rising.

The early failure of any partnership to take hold between Obama and Ryan, the chairman of the House Budget Committee, reflects the inability of the president to bridge the chasm between Democrats and Republicans in Washington — a goal at the core of his 2008 campaign.

The two men, for all their policy differences, have some similarities. Both are young — Obama just turned 51; Ryan is 42. Both are intelligent and policy-oriented. Each appears sure of himself and of the rightness of his views, though Obama is probably more pragmatic while Ryan is proudly, conservatively ideological.

And both are men in a hurry. Obama reached for the presidency just two years into his first Senate term, and Ryan is widely viewed — including by Obama, aides say — as someone with presidential ambitions.

In Obama's first year of managing the financial crisis and the recession he had inherited, he and Ryan had little contact. Republicans then were the minority in the House, and Ryan had no committee chairmanship from which to push his "road map" plan for deep tax cuts, an overhaul of federal entitlement programs and other changes he sees as essential to restoring the nation's economic strength.

But by the start of 2010, as the economy showed signs — prematurely, it turned out — of strong recovery and Obama began thinking beyond short-term economic-stimulus measures to reducing long-term annual budget deficits, he reached out to Ryan.

In January of that year, the president joined House Republicans at their annual retreat and pointedly referred to Ryan, the senior Republican on the budget committee.

"I think Paul, for example, head of the Budget Committee, has looked at the budget and has made a serious proposal," Obama said. "I've read it. I can tell you what's in it. And there are some ideas in there that I would agree with, but there are some ideas that we should have a healthy debate about because I don't agree with them."

Calling spending for Medicare, Medicaid and other public-health programs "the major driver of our long-term liabilities, everybody here knows," Obama said Ryan had "an entirely legitimate proposal" in his idea to transform Medicare into a voucherlike system that would pay current beneficiaries a capped amount to buy private insurance.

Ryan interjected that his proposed Medicare changes would not apply to people now receiving benefits or to those within a decade of doing so. Obama continued, saying the problem with the plan was that the payments would not keep up with the increase in health-care costs. At the close, the men shook hands and the president signed an autograph for Ryan's young daughter.

Earlier in that session, Obama also had singled out Ryan to make a point less about policy than about politics in Washington.

"The problem we have sometimes is a media that responds only to slash-and-burn-style politics," Obama said. "You don't get a lot of credit if I say, 'You know, I think Paul Ryan's a pretty sincere guy and has a beautiful family.' Nobody's going to run that in the newspapers, right?"

Mindful that some Republicans were threatened by intraparty rivals for being seen as too friendly toward him, Obama added to his antagonists' laughter, "And by the way, in case he's going to get a Republican challenge, I didn't mean it." Looking directly at Ryan, he said, "I don't want to — don't want to hurt you, man."

A year later, Republicans had taken control of the House, and Ryan of the budget committee, after a midterm campaign in which they had attacked Obama, accusing him of cutting Medicare by $500 billion over 10 years in his health-care law — reductions that were the same as in the Ryan plan, which he now was shepherding to House passage.

In April last year, days before the House vote on Ryan's budget, Obama outlined his deficit-reduction plan for spending cuts and revenue increases roughly along the lines of recommendations from a majority of members on his Bowles-Simpson fiscal commission from the previous December that Ryan, a commission member, opposed.

Not knowing that Ryan was in the front row, though he had been invited by White House staff, Obama flayed the Ryan budget. Ryan, in an interview, said he afterward told Obama economic adviser Gene Sperling that the president had "poisoned the well."

Soon after, when Obama called House Republicans to the White House to start budget talks, Ryan challenged him to stop the political attacks. Obama countered that Republicans were no slouches at attacks, even of his birthplace.