His candidacy was scorned by the Republican establishment, including Gov. Corbett, which favored Welch, a Chester County tech entrepreneur, among five candidates who battled for the GOP nomination to oppose Democratic U.S. Sen. Bob Casey in the fall.
Welch, in the late going, was trailing Sam Rohrer, a former 18-year state legislator from Berks County, in the battle for second place.
The Smith victory was a political blow to Corbett, who had gotten the Republican State Committee to endorse Welch with the idea that the youthful, high-energy contender (he's 35 and a Penn State engineering grad) from the Philadelphia region would be the party's strongest candidate against Casey.
Welch, of Malvern, was never able to raise much money beyond the $1 million he gave to his own effort. He struggled to get others' help, and he never had as much money as Smith.
Welch could not even hold his corner of the state solidly. Though well ahead in Chester and Delaware Counties, he was trailing Smith in Bucks, Montgomery and Philadelphia Counties.
Two other Republican candidates, decorated Vietnam War veteran David Christian, a businessman from Bucks County, and lawyer Mark Scaringi of Camp Hill, near Harrisburg, were far back in the running.
Smith, in a victory speech from Pittsburgh, promised to work for an end to "Obamacare," a bigger role for Pennsylvania coal in national energy production, and a flat tax.
"Shame on you, Bob Casey, for not standing up to Barack Obama and his out-of-control EPA," he said. Smith cast the race against Casey in national terms - as a battle for party control of the Senate.
Casey, seeking a second six-year Senate term, easily won renomination over Joe Vodvarka, owner of a coil-spring manufacturing shop near Pittsburgh, who ran as a foe of free trade with China.
In a statement, Casey said he wanted "to continue to serve as an independent fighter for the people of Pennsylvania. . . . We need to put more Pennsylvanians back to work, and unfair foreign trade is allowing countries like China to cheat."
Welch, in defeat, said the GOP should get behind Smith - whose priority, he said, must be to make himself better known in this corner of the state. "You have to get the Southeast to win," he said.
The GOP could get none of its established officeholders to take on Casey, who will start the fall campaign as the favorite. But his fortunes, to some extent, could be tied to Obama's. Casey was one of Obama's earliest and most important Pennsylvania supporters, and the two could be linked in how voters view the economy.
Democrats seemed to delight in the war of words between Smith and Welch over which had the strongest history of Democratic Party ties, an embarrassment for each in a GOP primary.
Smith was a registered Democrat for decades, a Democratic township supervisor, even a Democratic committeeman as recently as two years ago. He said he was always conservative, though. Welch registered as a Democrat from 2005 to 2009.
Rohrer, 56, who had little cash to work with, had hoped to work his way to victory quietly. He had a devoted following of antitax and tea-party activists, mostly in south-central Pennsylvania. He received nearly a third of GOP primary votes in his 2010 bid for governor against Corbett.
He predicted Smith's challenge to Casey will be an "uphill battle."
Contact Tom Infield at 610-313-8205 or email@example.com, or tinfield@Twitter.