In Tuesday's Republican primary for the U.S. Senate, Pennsylvania voters may be hard-pressed to find much ideological difference among the five candidates, who have sounded like a finely tuned quintet in decrying taxes and federal regulation.

The choice for voters then becomes, whose background and personal story hold the most appeal? On that score, the contest for the GOP nomination to oppose Democratic incumbent Bob Casey Jr. has had the discord of a rush-hour fender-bender on City Avenue.

Tom Smith, 68, who has a high school education and who worked his way to wealth in coal mining, has steered a few shots at fellow businessman Steven Welch ("a liberal") and former State Rep. Sam Rohrer (he voted for a legislative pay hike).

But the aggressive driver in the race has been Welch, the self-described "nerdiest candidate," a Pennsylvania State University engineering graduate who, at 35, has made riches of his own as a tech-entrepreneur and venture capitalist.

Welch has been on the attack with the ferocity of a candidate who appears to see himself trailing in the race but who believes he can catch up.

On Thursday, around the time of a cordial candidate forum in Northeast Philadelphia, Welch's campaign posted a YouTube video containing clips of Smith from previous events, and describing him as "rambling, incoherent," "insanely idiotic."

The video snippets, taken from longer public remarks, were meant to paint the folksy Smith as a bumpkin. As far as Smith is concerned, Welch can have the razor-sharp elbows approach. He views himself as the intelligent common man - a Mr. Smith who hopes to go to Washington, or, as he puts it, "just a farm boy that got misplaced in the coal mines and started my own business."

Even in the final weekend of the primary campaign, the GOP Senate race was attracting scant public attention, especially in the Philadelphia area, where TV advertising is the most expensive in the state.

Smith and Welch notably have attacked each other for past associations with the Democratic Party, a charge with heft in a polarized political climate. (Smith was a Democratic township supervisor and party committeeman; Welch was a registered Democrat and says he voted for President Obama in the 2008 Democratic primary.)

But whatever real oomph Tuesday's primary might have had was stripped away when former Sen. Rick Santorum exited the GOP presidential race, taking the media glare with him. A Pennsylvania showdown between him and likely nominee Mitt Romney would have drawn more voters to the polls. In a low-turnout primary, party stalwarts like those backing Welch may have an outsize influence.

There there has been little public polling in the GOP Senate race, but it seems to be competitive. Smith, Welch, and Rohrer all appear to have a shot, analysts say.

Smith, of rural Plumcreek Township in Armstrong County, has donated $4 million to his campaign, largely for TV ads. His edge is money; his pitch, business know-how.

"I think what I bring to the table is, I have been in business for 40 years one way or another," he said in an interview. "I know what it is like to balance a budget and run a business - and the regulatory burden."

Welch, of Malvern in Chester County, has donated $1 million of his own money. But his edge is the backing of Gov. Corbett and the Republican State Committee.

"I believe in the brilliance of America," Welch said. "I believe in the enterprise spirit of America, in which people can use their God-given talent and creativity to produce products and services people need - and that improves the standard of living of all Americans."

Rohrer, 56, of Elverson, Berks County, is the earnest policy wonk in the group. Though he served 18 years in the state House, he has fought the career-politician charge by saying he is the only candidate with a proven record on issues GOP voters care about - "pro-gun," "for life," and best known for his long effort to end school property taxes in Pennsylvania.

His edge is grassroots support from many of the same tea party and antitax groups he courted in his 2010 GOP primary campaign for governor against party favorite Corbett.

Rohrer got nearly a third of the votes in that two-way primary with Corbett. That could be enough to win in a five-way race.

Experience counts, he said.

"Do you expect a pilot with no experience to get into the cockpit and fly your plane? The idea that just having somebody new and different is the answer is simply not true."

The other candidates in the race, Marc Scaringi and David Christian, do not appear to have any distinct edge that could vault them into the top tier of candidates.

Scaringi, 41, of Camp Hill in Cumberland County, is a Widener University law school graduate who, with his wife, Melanie, operates a law firm in Harrisburg and at two satellite offices.

"A lawyer who wants to be a politician" - that's how he described himself at the Thursday forum in Northeast Philadelphia sponsored by the local Chamber of Commerce. He made his bones in GOP politics as an aide to Santorum in the 1990s.

"Freedom, personal liberty, independence, self-reliance, and the constitutional republic - I am very focused on restoring the Constitution and limited government," he had said.

For the 40ish-or-over voter from Bucks County, Christian's name on the ballot may look familiar. He ran twice as the GOP nominee for Congress against a Democrat, then-Rep. Peter Kostmayer, in 1984 and 1986.

Now 63, Christian has spent the intervening years in defense contracting. He owns Dac-Val Inc. in Philadelphia, a maker of ground-support equipment for the Navy.

In his youth, he was one of the most decorated soldiers to return from the Vietnam War.

"I have been a Ronald Reagan Republican all of my life, and I want to be exactly what Reagan was," he said.

Christian recalled a White House meeting in which he said President Reagan slapped him on the knee and declared, "We've got to cut regulations, we've got to cut taxes, we've got to cut the bureaucracy."

Watching from the Democratic side, incumbent Casey has yet to really begin his campaign. He has been content to watch the Republicans fight it out.

Several prominent Republicans in the state legislature considered running against Casey, but - out of apparent respect for his political strengths - backed out.

Casey knows, though, that he will have to fight Pennsylvania GOP contentions that he is the "most liberal" member of the Senate. And his fortunes could go up or down with those of Obama, who will lead the ticket in November.

Joe Vodvarka, Casey's Democratic primary opponent, had been taking time off from his coil-spring manufacturing shop near Pittsburgh when he could to campaign, riding around the state with his son behind the wheel.

Vodvarka talks about what he says is the destruction of American manufacturing because of low-cost goods from China, often made for American companies.

"All this stuff is made in China; that's why I'm running for the U.S. Senate," Vodvarka, 68, said. "The corporations have abandoned America. They are the ones who used to pay taxes and hire people. Now they go to third-world countries and pay people under $1 an hour."

As for the evils of taxes and government regulation, he'll leave that talk to the five Republicans on the ballot.

Contact Tom Infield at 610-313-8205, tinfield@phillynews.com, or follow @tinfield on Twitter.