Sen. Thad Cochran (R-Miss.) arrives at the Senate chamber for a vote on Capitol Hill . Cochran, 76, in Congress since 1973, is in the reelection battle of his career as the Senate primary in Mississippi. (AP)

After a string of humbling defeats in Republican primaries this spring, the tea party’s last best hope to oust a lawmaker is in Mississippi.

But things are not going well for the movement’s Chris McDaniel, who is challenging longtime senator Thad Cochran.

The race has been roiled over the past week by a bizarre incident in which a pro-McDaniel blogger was arrested in connection with taking an illicit photo of Cochran’s bedridden wife, Rose, who has dementia and lives in a nursing home. More arrests were made on Thursday, including a Mississippi tea party activist who is closely connected to McDaniel.

At first glance, the deeply conservative state’s Senate primary contest seems ripe for an upset. Cochran, 76, has served in Washington as long as McDaniel, a 41-year-old state senator, has been alive. A proud and prolific earmarker when senators were freer to send pork to their states, Cochran personifies the kind of free-spending Beltway broker that grass-roots conservative voters have often revolted against.

Yet McDaniel has been unable to put the race away to deny Cochran a seventh term. He has made a series of tactical errors, while Cochran and his establishment allies — well funded and prepared for battle — condemned McDaniel on the airwaves.

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There are few reliable public polls in Mississippi. Strategists in the state say the race has been close, with Cochran holding a slight advantage, although they sense that McDaniel had been gaining steam before the nursing-home episode.

Conservative groups — hoping for their first victory in knocking off an entrenched incumbent this year — are doubling down to prop up McDaniel in the run-up to the June 3 primary.

“This is it,” said political analyst Stuart Rothenberg, editor and publisher of the Rothenberg Political Report. “For all the early hype about the McConnell primary challenge, the Cochran race has always been the best opportunity for the libertarian-slash-tea-party folks,” he added, referring to Senate Minority Leader Mitch McConnell (R-Ky.).

Citizens United and ForAmerica will soon begin advertising in Mississippi, joining an array of conservative organizations backing McDaniel. Together, tea-party-aligned groups have spent nearly $2.4 million, compared with the $1.8 million that pro-Cochran groups such as the U.S. Chamber of Commerce have invested, according to Federal Election Commission data compiled by the Sunlight Foundation.

L. Brent Bozell III said his group, ForAmerica, has “decided to get behind McDaniel this week with a six-figure buy” focused on digital advertising.

“I’m not looking for scalps, but I am looking for conservative victories, and Cochran remains a top target,” he said. “He campaigns as a conservative and governs as a moderate.”

David Bossie, president of Citizens United, whose political arm has backed McDaniel since January, is ramping up his efforts. He said that beginning Thursday, he will spend $175,000 for statewide cable and broadcast advertisements — a significant buy for the modestly sized tea party group.

“This is about Mississippi voters, who want a conservative leader in Washington raising Mississippi’s voice to be with the Ted Cruz-Mike Lee part of the party, not part of the establishment and part of the problem,” Bossie said, referring to the combative senators from Texas and Utah.

The biggest spender has been the Club for Growth, which has been on the air for weeks and spent more than $1.7 million. The Senate Conservatives Fund has spent about $1 million, while other groups — including the Tea Party Patriots and the Madison Project — have been active, according to the Sunlight Foundation analysis.

The torrent of spending drew a warning from former Delaware congressman Mike Castle, who lost the 2010 GOP Senate primary to Christine O’Donnell, a tea-party-backed candidate who was crushed in the general election.

Castle said Cochran “needs to be careful since these tea party groups can bring in a lot of money in a rush. . . . Even if you’re more prepared than I was, and the surprise is sort of gone, you’ve got to pay close attention to what they’re doing.”

Cochran’s advisers and advocates insist the money is all for naught. Although they acknowledge the race is tightening, they also argue that Mississippians will not turn out a senator who, as a member and former chairman of the Appropriations Committee, has delivered time and again for his poor and rural state, such as after Hurricane Katrina ravaged the Gulf Coast.

“He can just accomplish so much more than Chris McDaniel, and I think people see a contrast in that we know we can count on Senator Cochran,” said Henry Barbour, a nephew of former Mississippi governor Haley Barbour (R) who is running Mississippi Conservatives, a pro-Cochran super PAC.

He recalled an exchange, reported in February by Politico, in which McDaniel told an audience at the University of Mississippi: “I’m not going to do anything for you. I’m going to get the government off your back, and then I’m gonna let you do it for yourself.”

In recent years, the party’s right-most wing has viewed Cochran suspiciously. In its 2014 scorecard, the American Conservative Union shows Cochran at 63 percent — one of the group’s worst ratings during the Obama presidency. That’s 31 points lower than McConnell, who fended off a tea party challenger this week, and 26 points lower than Sen. Pat Roberts (R-Kan.), who also faces a primary fight.

“It’s a critical moment for conservatism,” said ACU Executive Director Dan Schneider. “It will test whether conservatives can get rid of senators who become more liberal over time — and Thad Cochran is the king of that group.”

Cochran plans to tour the state by bus in the final days of the campaign and is seeking help from social conservatives who have been friendly with him for decades. National Right to Life, an antiabortion group, endorsed him Wednesday, saying he had advanced “vital pro-life public policies.” Stuart Stevens, a Mississippi native and former adviser to Mitt Romney, has been enlisted to guide Cochran’s media strategy.

Cochran, who has not run a competitive race since 1984, is relying heavily on the GOP political machines built by Haley Barbour and the state’s current governor, Phil Bryant. The U.S. Chamber of Commerce, which has spent $500,000 on the race, also is mobilizing its network of local chambers and small businesses to encourage their employees to support Cochran because it views him as an able recruiter of jobs to Mississippi.

Henry Barbour, noting tea party struggles in other contested primaries this year, said, “Candidates matter.”

“That’s what these groups that have hijacked the tea party need to learn,” he said. “That’s why Senator Cochran’s going to win and why Chris McDaniel’s going to lose.”

Still, some Washington party establishment figures quietly voice concern about Cochran’s vulnerabilities. One of them, who spoke on the condition of anonymity because he did not want to alienate Cochran, compared the genteel lawmaker to former senator Richard G. Lugar (Ind.), who lost to a tea party challenger in the 2012 election at the age of 80.

Lugar was felled by accusations that he had been in Washington so long that he lost touch with his state, selling his house in Indiana and living full time in suburban Virginia.

“Thad Cochran is a perfectly nice Southern gentleman and senator,” Bossie said. “But I don’t see him being a leader who will work against the status quo. His time has passed.”

That is the theme McDaniel wants to drive home in the final days of campaigning. But both contests have dragged the other into an ugly back-and-forth over the Rose Cochran nursing-home photo. Conservative blogger Clayton Kelly posted an online video containing the unauthorized picture and later took it down, but he was arrested last week and charged with a felony.

On Thursday, authorities also arrested Mark Mayfield, vice chairman of the Mississippi Tea Party and a McDaniel supporter, in connection with the case.

Although McDaniel’s campaign insists that it had nothing to do with the incident, Cochran’s team tried to create doubts by releasing a timeline of statements from McDaniel and his aides and showing how they changed their story.

McDaniel said the Cochran campaign is resorting to “shameful slander,” while Cochran spokesman Jordan Russell told reporters this week, “We’re in the middle of a race I think the likes of which Mississippi has never seen.”

Some tea party leaders recognize the difficulty in defeating the Mississippi incumbent.

In 2010, tea party favorite Joe Miller beat Sen. Lisa Murkowski (R-Alaska) in a primary, only to lose to her in the general election, in which she ran as a write-in candidate. Miller, who is running again this year, said in an interview that it is much harder for contenders like McDaniel.

“The ruling class of the Republican Party has become much more explicit in its attacks on conservatives than they used to be,” he said. “They’ve designed a better mechanism to keep their power.”

Tom Hamburger and Matea Gold contributed to this report.