Republican presidential candidate, former Mass. Gov. Mitt Romney, accompanied by House Budget Committee Chairman Rep. Paul Ryan, R-Wis, greet people and hand out submarine sandwiches during a campaign stop at a Cousins Subs fast food restaurant in Waukesha, Wis., April 3, 2012. (Steven Senne/AP Photo)

ABC News' Shushannah Walshe and Emily Friedman report:

Mitt Romney and Paul Ryan may not see eye to eye on everything. The presumptive GOP nominee has not embraced every aspect of the House Budget Chairman's signature Medicare proposals, for instance. But he has embraced Ryan as his running mate.

Over the past six months the two men seem to have developed a close personal relationship. Mitt Romney is famous for sometimes having a hard time connecting on the stump, but that's not the case with the 42 year old congressman. They seem to have a similar head for data and policy and a comfort level that has turned into a fast friendship from their time campaigning together since he endorsed Romney in March.

In Wisconsin in on primary day the two stopped by a sub shop to hand out sandwiches. The two stood behind the counter and doled out sandwiches to supporters, playing off one another and posing for photos.

On April Fool's Day in a video that went viral, Ryan was involved in an elaborate prank with Romney. The former Massachusetts governor bounded in with a smile on his face expecting a crowd and he got an empty room. The two fell apart laughing and the body language reveals an obviously relaxed, buddy relationship.

It's one that over the next three months to November will obviously be closer and that comfort level may have greatly factored in to Ryan's selection.

On their Wisconsin tour together, Ryan boasted that his picks for the NCAA tournament were so good, he knew he was right about picking Romney.

"Now, I'm feeling pretty good about my picks these days, because when I filled out my NCAA brackets, I picked Kentucky against Kansas in the championship game," Ryan said. "I did have Marquette and Wisconsin going pretty far. You know, everybody made it to the Sweet Sixteen, but I did pick Kentucky to win, and man, those guys really dominated last night, but when I filled out my ballot here in Wisconsin, I picked who I think is gonna be the next President of the United States. I picked Mitt Romney."

At another campaign trail fast food stop, the two stopped at Culver's, a Wisconsin staple. Ryan joked with Romney that he had to order a butter burger because it's what you eat in Wisconsin. The two, both tall and fit with dark hair, even look somewhat alike, and seem to have a brotherly relationship.

When the two were on the stump together, the relationship was obvious. Romney was comfortable with the congressman taking the stage and he would even sit down to listen during town halls when Ryan was talking. It was a step he didn't take with the other potential running mates during their auditions.

"What I see in Mitt Romney are the kinds of skills, tools, character attributes that you need in a leader! He makes decisions. He doesn't pander," Ryan said at the Reagan Library in May. "He is going to beat Barack Obama and I think that we are going to save this country."

Romney famously does not like to travel without his wife, Ann, but advisers who saw Romney and Ryan together on the stump noticed he was similarly happy and relaxed.

Ryan is already being targeted by Democrats because of his controversial ideas to address the deficit, but in an interview with the Milwaukee Journal-Sentinel's Craig Gilbert, Ryan indicated that if he were picked, he'd be ready for the fight.

"I want that debate . . . I'm really eager to have that debate. That's what we're here for. The reason I want that debate is they have to falsify to make their claims (about his budget)," Ryan said in the interview with Gilbert. "They have no alternative, no solution. All they have is, 'Let's stick with status quo.'"

He also is not shying away from his budget blueprint.

He recently told George Stephanopoulos that the election will turn on the economy and health care.

"This election is a choice of two futures: Do you want a government-centered society and a government-driven economy and government-rationed health care? Or do you want the American opportunity society with a safety net, a free economy, economic freedom, personal liberty?" Ryan said on 'This Week' July 1 . "That's what we want. That's the American idea. We have one more chance as a people to get that back, and that chance is going to come on November the 6th."

ABC News' John Parkinson and Michael Falcone contributed to this report.