ST. LOUIS  • Missouri primary voters set the stage Tuesday for a heavyweight showdown, with Republican Roy Blunt and Democrat Robin Carnahan each securing a place on the fall ballot for U.S. Senate.

The outcome, which came as no surprise, officially secured a matchup of Missouri's most prominent political families that has been building for months.

Other races decided Tuesday offered more suspense. Tom Schweich, the well-funded former U.S. State Department official seeking the GOP nomination for state auditor, defeated state Rep. Allen Icet, R-Wildwood, for the right to face incumbent Democrat Susan Montee in the general election.

Icet is chairman of the influential budget committee in the state House of Representatives, but it was Schweich — with help from prominent Republican donors — who barreled his way to a victory with a robust campaign account.

"It has been an exciting campaign, but the people of Missouri have spoken," Icet said in a concession to supporters. "I encourage Republicans to get behind Tom as he attempts to restore fiscal responsibility to the office of state auditor."

A law professor, Schweich gained the inside track for the nomination when he agreed to run for auditor instead of challenging Blunt for the U.S. Senate nomination. That deal triggered endorsements from top GOP officials such as Lt. Gov. Peter Kinder, who funneled more than $200,000 to Schweich's campaign.

Turnout was predictably light Tuesday, with sweltering temperatures perhaps helping to keep voters away. In St. Louis County, almost one in four voters turned out; while not quite one in five city voters cast ballots.

Not that there weren't intriguing matchups. A pair of heated state Senate races generated the most attention in St. Louis County. State Rep. Maria N. Chappelle-Nadal defeated two fellow state representatives and the former mayor of University City for the Democratic nomination in the 14th District, which stretches from Delmar Boulevard to past the airport. She will be unopposed in the general election.

In another battle between veteran Democrats, county Councilwoman Barbara Fraser easily won the Democratic nomination in the 24th Senate District, which covers Clayton and Creve Coeur, over former state Rep. Sam Page. Fraser will face Republican John Lamping, an investment adviser from Ladue, in the general election.

In St. Charles and Lincoln counties, Republican voters in the 2nd Senate District backed Sen. Scott Rupp, who held off a primary challenge from Rep. Cynthia Davis, who is known for strong socially conservative views and a penchant for making controversial comments.

She tried to overcome Rupp's fundraising advantage by accusing Rupp and others in the Republican Senate majority of not always remaining true to GOP ideals. For example, she attacked Rupp's vote in July for tax incentives which were passed to try to keep a Ford plant near Kansas City.

"We won because we have a four-year record of getting results for Missouri taxpayers," Rupp said.

In St. Louis, city Democrats in the 4th Senate District primary handed incumbent Joe Keaveny a fresh term over former St. Louis police Sgt. Jim Long, marking a win for City Hall leaders and supporters of a locally controlled police force.

Long, also a former chairman of the Police Pension Board, had been the champion of police and firefighter unions who were seeking to oust Keaveny from the seat formerly held by Jeff Smith, who is in federal prison for his role in a political cover-up.

Keaveny was put into the post by city Democrats last year after Smith resigned.

The police and fire unions have worried that Keaveny would continue to fight for a locally appointed Police Board, instead of the current governor-appointed structure. Some fear that would mean an increase in scrutiny over rising fire and police pension costs.

The primary results also set up a November congressional election between St. Louis Democrat U.S. Rep. Russ Carnahan, Robin Carnahan's brother, and Republican lawyer Ed Martin, who defeated two rivals in the GOP congressional primary.

In the U.S. Senate contest, Carnahan, 49, Missouri's secretary of state, had only token primary opposition. Carnahan's father, Mel Carnahan, served as governor and died in a plane crash running for the U.S. Senate. Robin Carnahan's mother, Jean, was appointed to the seat and served for two years.

Blunt, 60, a former Missouri secretary of state who has served seven terms in Congress representing southwest Missouri, brushed back a spirited run from state Sen. Chuck Purgason, R-Caulfield.

A Post-Dispatch/KMOV-TV (Channel 4) poll last month showed Blunt with a lead in a matchup against Carnahan. The poll indicated voters were frustrated with the economy and the job performance of President Barack Obama.

At their election night parties, Carnahan and Blunt hit on familiar themes in their victory speech. Carnahan, speaking from Kansas City, indicated she would continue to hammer Blunt as a Washington insider. Blunt, who held his election night party in St. Louis, vowed to make the race about the role of the federal government.

"This is a campaign about whether the people are bigger than the government, or the government is bigger than the people," Blunt said.

Carnahan, asked if the election will be a referendum on the president, deflected the question. "I am not surprised Congressman Blunt is trying to run against somebody else," she said. "I would be embarrassed by his record in Washington as well."

Those seeking an insight into voter sentiment will be hard-pressed to find it in Tuesday's results. Voters approved Proposition C, a challenge to the health care law passed by Congress this year, by a wide margin. But Purgason, who had the support of many Tea Party activists, barely put a dent in Blunt's lead.

Purgason said he was disappointed in the result, but he thought he had moved the campaign in the right direction. He did not commit to supporting Blunt in the November election, though he didn't rule it out.

Two incumbents were rejected in St. Louis state House races. Democrat Penny Hubbard beat incumbent state Rep. James Morris in the primary. The seat was previously held by Hubbard's son, Rodney. Democrat Karla May edged her party rival, state Rep. Hope Whitehead, who won the seat in a special election earlier this year.

Susan Carlson, a University City lawyer, won a crowded Democratic primary for the right to replace State Rep. Rachel Storch, who did not seek another term.