Six weeks before their final showdown, Missouri’s three major Republican candidates for the U. S. Senate are kicking their battle for the GOP nomination into high gear with more endorsements – and more attacks against each other.

The trio – U.S. Rep. Todd Akin, St. Louis businessman John Brunner and former state Treasurer Sarah Steelman – also are getting some help.  The Missouri chapter of Americans for Prosperity, a national fiscal conservative group, is launching a new round of attacks this week against the GOP’s prime target, U.S. Sen. Claire McCaskill, D-Mo.

The group long has attacked McCaskill as a big-spending Democrat. Americans for Prosperity was conducting robo-calls Sunday to prospective Republican voters to promote an event Tuesday at the Drury Inn at Interstate 44 and Highway 141. The event will unveil four “command center vans’’ to drive around disparaging McCaskill’s Senate record.

Americans for Prosperity also announced that it is launching a new statewide TV ad on Tuesday that attacks McCaskill.

McCaskill and top Missouri Democrats, including Secretary of State Robin Carnahan, have attacked such outside help. At last Friday's Jefferson-Jackson Dinner, Carnahan also accused the trio of Republican rivals of trying to "out-crazy" each other.

Steelman, Brunner and Akin so far have ignored such Democratic jabs.  Instead, the trio -- while still civil in their debates -- increasingly are seeking to set themselves apart in their appeals to likely GOP voters in the Aug. 7 primary.

 On Monday, Steelman and Akin swiftly weighed in with reaction to the U.S. Supreme Court's decision upholding key parts of Arizona's strict provisions aimed at curbing illegal immigration -- a popular measure among conservative Republicans.

"I was glad to hear that the U.S. Supreme Court upheld Section 2B, empowering Arizona's law enforcement to check the immigration status of the people they stop," Steelman said in a statement.

"States facing an onslaught of illegal immigration need to be able to pass and enforce laws to protect their citizens. I am pleased that the Supreme Court affirmed the vital role of 10th amendment in the constitutional framework established by our founders."

Akin's initial reaction was more tempered. "This decision is a mixed bag," he said in a statement. "Illegal immigration hurts our country in numerous ways and it is sad that states are doing more to enforce our immigration laws than the federal government."

Later, Akin issued a revised statement in which he said he was concerned because "despite dissenting opinions by some of our constitutional justices, the court struck down three seemingly good provisions of the Arizona law.  The fourth provision, surprisingly the most controversial of the group, was unanimously upheld."

“I believe that this is, in part, a victory for the conservative movement," Akin added.  The congressman also cited his latest bill that seeks to prevent illegal immigrants from collecting federal benefits, including Social Security.

(Start of update) Later, Brunner also issued a statement saying, "I am encouraged by the justices’ decision to uphold the provision that enables law enforcement officers to enforce Arizona’s state laws."

Like his rivals, Brunner also criticized McCaskill and President Barack Obama, accusing them of showing an "inability to effectively secure our nation's borders..."

"I will oppose any effort to grant amnesty to those illegal immigrants that have broken the law. At the same time, our legal immigrants should be protected by streamlining the application process," Brunner added.(End of update)

Steelman’s scheduled stops on Sunday included the annual picnic in Carondelet Park of the Republican Central Committee in St. Louis.

However, she apparently could not make the event and sent a representative instead, her campaign confirmed Monday. Brunner also had a surrogate at the event.

Although the city's Republicans are dramatically outnumbered, their votes remain highly sought in what could be a close statewide primary.

 Steelman’s urban visit contrasts with the trio’s largely rural focus – pro and con -- during the past week.  Brunner, for example, has highlighted his latest endorsement from a rural farm group, the Missouri Pork Association.  In recent weeks, he also has collected the backing of the Missouri Soybean Association.

Brunner is particularly promoting the pork association's support because the group rarely picks sides in a primary.

“We are endorsing in the primary for the first time in our association’s history because of the vital nature of this election and the strength of John Brunner as a candidate,” said Don Nikodim, executive vice president of the Missouri Pork Association, in a statement. “Washington is broken and its regulations are harming our industry. John Brunner, with his business background, is the best candidate in this race to fix Washington and ensure our pork producers have the best opportunities to succeed.”

But the endorsement’s timing also came as Brunner was under attack last week over a donation that his family’s Brunner Foundation made a few years ago to the Humane Farming Association, an animal-rights group dubbed by Steelman as “radical.”

Brunner and his wife are the foundation’s trustees. He says the foundation’s $10,000 donation in 2007 to the animal-rights group actually came from one of his children. The family has a policy of allowing the children to designate foundation money to their own favorite charities, Brunner said.

“I am extremely proud of my children, but, as with most families, I do not always agree with their choices or decisions,’’ Brunner said in a statement issued after the controversy broke. “Regrettably, there are organizations that far too often use deceptive rhetoric and tactics to deceive well-meaning people into supporting their far more dangerous agendas. But, let me be perfectly clear, I, personally, have never supported the Humane Farming Association.”

Steelman, however, contends that Brunner can’t disavow donations that his own family’s foundation makes. “It seems that we can now add agriculture to the growing list of issues Brunner has either no clear vision or perhaps whose views are opposed to Missouri farmers,” her campaign said in a statement.

Steelman, by the way, also continues to be under attack from Brunner, who is seeking to portray her and Akin as big-spending Republicans. The prime targets are Steelman’s budget votes while a member of the state Senate, and Akin’s votes to increase the federal debt ceiling. Both contend that Brunner is mischaracterizing their votes.

Brunner disagrees. He has launched two new attack web sites -- www.SteelmanFacts.com and www.AkinFacts.com – to continue to hammer his rivals.

As for Akin, his latest focus is primarily national. Akin is highlighting his opposition to President Barack Obama’s recent executive order to halt deportation of young illegal immigrants, age 30 or younger, brought into the country while children.

Akin announced last week that he has introduced a bill to require the federal Department of Homeland Security “to verify that an individual is not an illegal alien prior to becoming eligible for federal entitlement programs,” including Social Security, Medicare and Medicaid.

Akin also is among 66 members of Congress who have co-signed a letter "urging Secretary of Defense Leon Panetta to investigate a pattern of hostility toward faith in the United States Air Force."

"The letter calls on Secretary Panetta to issue clear Department of Defense policy guidance, consistent with our Constitution, to preserve the place of religious expression in the military at large," Akin's office said in a statement.

Akin also has been the most outspoken among the three in asserting, along with some other congressional Republicans, that the Obama administration is at fault in the controversial “fast and furious’’ gun-running program that some federal law enforcement authorities had conducted since 2006 in a failed effort to track down Mexican leaders in the illegal drug trade.

Akin’s latest efforts appear likely to be popular with the tea party crowd that all three Republicans are wooing.