An earthquake in Mexico sent minor shock waves as far as Arizona last week, but the reverberations of last month’s health care reform vote are likely to persist far longer in the state’s 8th district.

A field of four Republicans are hammering sophomore Rep. Gabrielle Giffords (D) on her “yes” vote, and the national GOP is hoping the issue will be a determining factor when voters decide whether to send the Congresswoman back for a third term in November.

“She’s someone that has banked on being independent and moderate,” National Republican Congressional Committee spokeswoman Joanna Burgos said. “Unfortunately for her, that’s just not going to be possible this time.”

Giffords, a Blue Dog Democrat who is among the targets for defeat being promoted by former Alaska Gov. Sarah Palin (R), is facing a tough fight — especially since Tucson native Jonathan Paton (R) stepped down from the state Senate to challenge her.

Paton and his GOP primary counterparts, including upstart tea party candidate Jesse Kelly, a 28-year-old local construction manager, have been relatively unified in their sustained criticism, calling Giffords a Washington, D.C., insider beholden to her party’s leadership.

“Her actions are more in line with [Speaker] Nancy Pelosi [D-Calif.] than what I think the core of this district is,” Paton said during a recent GOP primary debate on a local PBS affiliate.

Giffords’ campaign counters that the 8th district — which contains part of Tucson and most of southeastern Arizona and where Giffords succeeded moderate Rep. Jim Kolbe (R) in 2006 — is too moderate for the GOP’s conservative rhetoric, namely the “repeal and replace” mantra to which Kelly and Paton subscribe. “We’re seeing a lot of extreme language and a lot of extreme positions that are, in our opinion, pretty out of touch with the voters of this district,” said Anne Hilby, communications director for Giffords’ campaign.

Giffords was selling her vote over recess, giving PowerPoint presentations to constituents and telling local journalists, “It’s a human rights issue, it’s a civil rights issue.”

Her spokesman, C.J. Karamargin, said voters have been responsive.

“I think there’s a silent majority out there,” he said. “I think those people were appreciative of the Congresswoman’s vote on health care reform, and they will show their appreciation in November.”

But Paton’s early fundraising has turned heads. He pulled in more than $500,000 since declaring in mid-January, according to his campaign.

“Jonathan has that magic mix of both support financially and the support on the grass-roots level,” Paton Communications Director Daniel Scarpinato said. “He’s uniting conservatives.”

But Giffords is no fundraising slouch. She pulled in more than $490,000 in the first quarter, bringing her campaign war chest to a considerable $1.9 million. Paton’s cash-on-hand total is about $500,000, Scarpinato said.

Kelly, who has been in the race for about a year now, trails Paton with about $100,000 in first-quarter fundraising and about $200,000 in cash on hand. Two other little-known GOP contenders, Brian Miller and Andy Goss, trail distantly in the money chase.

The GOP field is interesting in that all four contenders served in the current wars: Paton, Kelly and Goss in Iraq and Miller in Afghanistan. This could play well in veteran-heavy areas of the district, such as Green Valley and Sierra Vista, in-state political observers said. But Giffords also plays up her military connection, frequently calling herself a military wife (her husband is astronaut and Navy Capt. Mark Kelly — no relation to Jesse Kelly).

As the primary frontrunner, Paton is trying to position himself as the inside-outside candidate — a Washington outsider, but one with enough of a conservative track record and legislative experience at the state level to make an impact in Congress.

Paton touts his statewide votes to strengthen border security, a seminal issue in Arizona that has only been made more poignant by the March slaying of a popular rancher in which illegal immigrants are the top suspects.

“People, they certainly do not like what they see in Washington,” Paton said during the primary debate. “But they also want to see someone that has more than rhetoric, that actually has experience to get the job done.”

The comment is a dig at his GOP foes, none of whom have legislative experience, and especially Kelly, who is nipping at Paton’s heels by running to his right. Kelly ambitiously attacks Paton’s state voting record and his endorsements, among other issues.

“He voted for [then-Arizona Gov.] Janet Napolitano’s ’08 budget that bankrupted the state,” Kelly told Roll Call. “And when you have the open borders amnesty group of [Rep.] Jeff Flake [R-Ariz.], Jim Kolbe and [ex-Majority Leader] Dick Armey [Texas] standing behind you, it’s hard to claim you’re anything besides the establishment candidate.”

Kelly’s tenacity has helped him pick up some big-name conservative endorsements, such as controversial but popular Maricopa County Sheriff Joe Arpaio, House Republican Conference Chairman Mike Pence (Ind.) and Rep. Michele Bachmann (Minn.).

Paton’s camp, however, contends that, “If he’s the establishment candidate, he certainly doesn’t look like it or feel like it,” Scarpinato said. “He has spent his entire life here [in Tucson]. He lives a mile from the house he grew up in and the hospital he was born in.”

The NRCC has elevated Paton and Kelly to “On the Radar” status in its “Young Guns” program, and the committee won’t choose sides but rather let the primary determine the nominee.

Democrats, though, say a heated primary could work to their advantage by causing unrest in the GOP — especially since the primary is in August, only 69 days before the general election.

“I think most people would probably tell you that Paton will be victorious at the end of the day,” said a Democratic Congressional Campaign Committee operative familiar with the race. “But he may be roughed up a bit in the primary and spend a bit more in the primary than he would like to.”

If Paton emerges the victor, the operative said, Democrats are ready to attack his vote for the current Arizona budget, which includes deep cuts to social programs, and his past post as a lobbyist for the payday loan industry.

Still, all four GOP primary candidates are collegial, have pledged to support the victor and frequently say the primary will make whomever emerges a stronger candidate against Giffords.

“Most years I would say, ‘Yes, that would hurt,'” Kelly said, referring to the late primary. “I do not like our late primaries in Arizona. But Gabrielle Giffords has shown herself to be so out of touch with the voters … this year, it’s not going to matter.”

If Kelly upends Paton, it will resemble 2006, when former state lawmaker Randy Graf channeled populist anger to take out establishment-backed state Rep. Steve Huffman in the GOP primary. But Huffman was more moderate than Paton, specifically on abortion and stem-cell research funding.

On paper, Paton more resembles Tim Bee, the former Arizona Senate president who challenged Giffords in 2008 (incidentally, all three went to the same middle school). Paton, however, seems to have the intangibles that Bee didn’t — namely more charisma and a better-run operation. Giffords defeated both Bee and Graf by 14 points.

The challenge for Giffords now, observers said, is that while she may have gotten a bump in ’06 and ’08 by riding the wave of anti-Bush sentiment, this year’s tone is anti-incumbent.

“The real difference is that in 2008, Gabrielle Giffords had not made the career-defining votes that she has made since,” Scarpinato said, giving another nod to the significance of the health care vote. “It was more difficult to draw that distinction. We don’t have that problem.”