New ads, new attacks and a big name endorsement. And that’s just Tuesday.

Missouri’s three-way Republican battle for the U.S. Senate is attracting more national attention -- and national money --  that underscore the national stakes of the contest among St. Louis businessman John Brunner, former state Treasurer Sarah Steelman and U.S. Rep. Todd Akin, R-Wildwood.

Even the incumbent they’re seeking to oust – U.S. Sen. Claire McCaskill, D-Mo. – got into the act on Tuesday with blanket attacks against all three.

(Start update) And Tuesday night, the national Majority PAC -- a SuperPAC with ties to Democratic Senate leaders -- unleashed a new ad in Missouri that targets only Brunner.

The ad attacks Brunner's business record and repeats some of the same assertions made by his two Republican rivals, especially Steelman.

Even so, the ad is highly unusual because it marks a rare instance where a national party is targeting just one state candidate among several fielded by the opposing party.  The Majority PAC's other prime targets in other states, for example, all appear to be Republican candidates who already have won their primaries and secured the GOP nomination.

Brunner's campaign contends that the Majority PAC's entrance into Missouri's already heated GOP primary signals that McCaskill and her allies are most afraid of Brunner, should he win the Aug. 7 primary.

"We believe this is unprecedented,'' said Brunner's spokesman Todd Abrajano late Tuesday.

SuperPAC ads are independent expenditures, so the Majority PAC -- run by Senate Majority Leader Harry Reid and other Senate leaders -- cannot coordinate or confer with McCaskill.

McCaskill, of course, is very familiar with SuperPAC ads. An estimated $8 million in GOP-aligned attack ads already have been run against her.

There is a difference between the Majority PAC and the GOP-allied groups targeting McCaskill:  the Majority PAC's donors are publicly identified, while the bulk of the SuperPACs attacking her are using a provision of the tax code that allows them to keep their donors secret.

McCaskill campaign asserted Tuesday that the three Missouri Republicans running for the U.S. Senate have generally opposed a measure, called the DISCLOSE Act, that would require that all SuperPACs identify their donors.

The Majority PAC ad appears to be running in St. Louis, Kansas City and Springfield -- the state's largest TV markets, Brunner's campaign said late Tuesday.

Brunner campaign manager Jon Seaton said in a statement, “In a truly unprecedented political move, (Senate Majority Leader) Harry Reid’s Super PAC is meddling in Missouri’s Republican primary, misleading Republican voters by attacking front-runner John Brunner.  Without question, Harry Reid and Washington Democrats know that the Senate majority hangs in the balance, and that John Brunner is the one candidate who will defeat Claire McCaskill in November." (End update)

The ad capped a day already filled with Senate news.

Former Alaska Gov. Sarah Palin, a magnet for social conservatives, announced Tuesday afternoon that she was endorsing Steelman. And the endorsement tied in with Steelman’s continued focus on rural Missouri.

 “Sarah has run a grassroots campaign the way it should be run – handshake by handshake, voter by voter, farm to farm, not TV ad after TV ad,” said Palin in a statement. “As a former state treasurer and state senator, Sarah has often had to stand up to the political establishment – even those in her own party – to stop out of control spending and the destructive debt it leaves in its wake.”

The endorsement brought a swift response from Brunner’s campaign, which had been sparring with Steelman and Akin this week over their latest ads. Each claimed the rival’s spot is inaccurate and misleading.

Said a Brunner spokesman: "We are surprised that a conservative like Sarah Palin would endorse a pro-labor, pro-trial lawyer, pro-tax, anti-business candidate like Sarah Steelman who clearly is an unreliable conservative. Gov. Palin must surely be unaware of Steelman's anti-conservative record.”

Ad wars escalate

Brunner has taken a similar approach in his latest ad, which attacks Akin and Steelman.  The ad cites her votes in the state Senate in 2003 in favor of a bipartisan bond issue, set up to help the state balance its budget during a few lean years. Brunner claims the bond issue put the state further in debt; Steelman notes that the Republican-controlled General Assembly had crafted the package.

As for Akin, Brunner’s ad cites his comments in a 2009 interview with the Beacon in which Akin said he planned to introduce a bill to require people to purchase insurance – an idea promoted at the time by conservative groups. Akin has said he dropped the idea after he looked more into it. He has noted that conservative groups have done the same.

Akin has countered with an ad featuring former Arkansas Gov. Mike Huckabee, another conservative favorite, who vouches for Akin’s conservative credentials.

Steelman’s new ad also has infuriated Brunner’s campaign. The ad alleges that Brunner was sued for back taxes, and questions his business skills while running Vi-Jon, a family-owned business until it was sold to an equity firm in 2006.

Brunner points out that the suit was against Vi-Jon, not him, and that all taxes were subsequently paid.  The ad doesn’t mention that Steelman also has come under fire over late property tax payments.

(Start update) Late Tuesday, Brunner issued a lengthy statement -- entitled a "fact sheet" -- that sought to challenge various assertions made in the Majority PAC ad.

Among other things, Brunner's campaign says the ad mischaracterizes $195 million in debt that Vi-Jon acquired during a merger with a competing company, Cumberland Swan, and wrongly implies that Brunner was still in charge when Vi-Jon's credit rating was downgraded earlier this year.

The Brunner camp also disputes the ad's portrayal of  Brunner's admitted misstep in the early 1990s, when he says he expanded Vi-Jon too quickly, and had to lay off some workers and sell off a portion of the company.  According to a Brunner ad about the episode, Vi-Jon later rebounded, and its business dramatically improved. (End update)

The Majority PAC's airing in Missouri's urban markets also contrasts with the recent rural focus of all three Republican candidates -- Brunner, Steelman and Akin.

All three have been directing their ads and literature in outstate Missouri, which is heavily Republican and could provide the deciding votes on Aug. 7.

McCaskill blasts GOP opposition to farm bill

McCaskill also directed her focus Tuesday at rural Missouri, by holding a conference call with reporters aimed at highlighting her support for the reauthorization of the federal farm bill -- and her GOP rivals' opposition.

McCaskill contended that Missouri's cattle ranchers are close to financial ruin, until the U.S. House passes the version of the farm bill that already has been OKed by the Senate.

The federal provisions intended to help lifestock producers have expired in recent months, she said, so a temporary extension of the still-in-force parts of the law wouldn't help them.

McCaskill was joined by rancher Stanton Thompson, of Lafayette County, who told how the drought is destroying pasture land used to feed his cattle.

McCaskill then took aim at Akin and Steelman, who have stated their opposition to the bill. "I am disappointed in my opponents,'' she said, for taking a position that "makes me believe they don't understand agriculture in Missouri."

She noted that U.S. Sen. Roy Blunt, R-Mo. and the son of dairy farmers, voted in favor of the bill.

Akin and Steelman have said their opposition stems from the bill's provisions regarding the federal food stamp program, which they want trimmed more than what the bill seeks. Some Democrats decry the bill's current cuts, while McCaskill said it was an adequate compromise.

Brunner has said that the food stamp program needs to be taken out of the farm bill, so that the issues could be dealt with separately. But he has not said whether he would support or oppose the various provisions. McCaskill accused him of "hiding behind talking points and fancy consultants."

McCaskill contended that the trio is bending to the will of tea party groups who oppose the farm bill for various reasons, including food stamps and the longstanding farm subsidies. Republicans in the U.S. House have said the split in their ranks goes beyond the tea party.

In any case, expect the fight to be featured in some way in the next round of TV ads in Missouri's U.S. Senate contest. Expect to see references to Palin as well.