FILE - U.S. Sen. Kyrsten Sinema, D-Ariz.,

U.S. Sen. Kyrsten Sinema, D-Ariz.

U.S. Rep. Kyrsten Sinema has competed in three triathlons and regularly pushes through muscle pain and fatigue during the spin cycle classes she teaches on Capitol Hill. But the Arizona Democrat is now facing a different type of grueling test: whether she will buck the progressive wing of her party and stand by her vow to uphold the Senate filibuster even if it imperils top Democratic priorities she supports.

Arizona Republican lawmakers aren’t making it easy. The entire GOP delegation in the state legislature is ramping up pressure on the first-term senator to keep her pledge. Last week, they signed a letter calling on Sinema to protect the filibuster and to refuse any Democratic efforts aimed at weakening it.

“We are grateful thus far you have stood up to the pressure within your own party to weaken or eliminate the filibuster,” the GOP legislators wrote in a letter provided to RealClearPolitics. “It is rarely easy or popular to go against one’s own party; however, we respect your conviction to stand firm to protect the rights of the minority party by defending the filibuster.”

Among the signatures of every GOP Arizona state legislator are those of Speaker of the House Rusty Bowers, Senate President Karen Fann, and Rep. Jake Hoffman, who spearheaded the effort.

The Republican push comes just days after Al Sharpton, civil rights leaders and voting rights advocates said they planned to launch their own pressure campaign accusing Sinema and Democratic Sen. Joe Manchin of “supporting racism” with their continued backing of the filibuster. Sharpton and other Democrats are ratcheting up their battle against the parliamentary maneuver, which stands in the way of their top legislative priority, S.1, a sweeping voting rights measure. (The House version, H.R. 1, has already passed.)

“The pressure that we are going to put on Sinema and Manchin is calling [the filibuster] racist and saying that they are, in effect, supporting racism,” Sharpton told Politico last week. “Why would they be wedded to something that has those results? Their voters need to know that.”

Many Democrats are attacking the filibuster as an outdated relic that was used to stall civil rights legislation in the late 1950s. But Democrats heavily relied on it during President Donald Trump’s tenure to block his agenda, including construction of the border wall, as well as to impede passage of the CARES Act last year until Republicans agreed to a $600 weekly federal unemployment supplement, and again in September and October to block Republicans from passing more coronavirus relief before the November election. In 2017, some 30 Senate Democrats signed a letter promising to preserve the right of the chamber’s minority to delay or block legislation.

Support for the filibuster now hangs by a two-vote thread. Sinema and Manchin have repeatedly said they want to keep it in place while other Senate Democrats who supported the rule in the past have changed their tunes. Majority Leader Chuck Schumer and Chris Coons, a close Biden ally, say they now view eliminating the filibuster as a way to break partisan gridlock on President Biden’s agenda. The rule essentially requires most legislation to receive 60 votes to pass the Senate. Manchin’s and Sinema’s stance are key to its fate with the chamber split 50-50 and the deciding vote going to Vice President Kamala Harris.

New cracks, however, are forming in Manchin’s once solid filibuster support. The folksy centrist in recent weeks has signaled a willingness to make using the filibuster more “painful” – requiring talking filibusters by eliminating the custom of simply threatening one. But Sinema has remained a stalwart supporter, stating through a spokeswoman in January that she’s “not open to changing her mind.” In February, she doubled down, saying she would even vote to strengthen the filibuster by reversing previous changes that jettisoned the 60-vote threshold for judicial nominations.

Reached by RCP on Monday, a spokeswoman asked for a copy of the letter from the Arizona Republicans but did not provide a response to it.

Sinema, once a Green Party member known for wearing a pink tutu as an antiwar activist, has transformed herself into a centrist Democrat in recent years through a careful metamorphosis. That political evolution helped her become the first Arizona Democrat elected to the Senate from the state in a generation. She tacked to the center to capture an open seat in 2018 against Martha McSally, the first female U.S. military pilot to fly in combat who, after losing to Sinema, was later appointed to Sen. John McCain’s seat after his death. In November, Democrat Mark Kelly, a former NASA astronaut who commanded the second-to-last space shuttle mission, beat McSally in a tight race to formally fill McCain’s seat.

To say that McCain, who represented Arizona in Washington for 37 years and defined what it means to be a political maverick, still looms large in state politics is putting it mildly. In 2017, his iconic thumbs-down vote on a GOP measure repealing the Affordable Care Act was the coda of his political career. Democrats hailed him as a hero while many Republicans labeled him an apostate after years of McCain’s harsh criticism of the health care law.

In early March, Sinema gave a nod to McCain when she voted against Sen. Bernie Sanders’ effort to include a $15 minimum wage increase in the $1.9 trillion coronavirus relief package. But it wasn’t as decisive as McCain’s “no” vote on Obamacare: She was joined by seven other Senate Democrats and all Republicans. Still, many of her fellow Democrats were enraged, saying McCain’s stance prevented harm to the less fortunate who relied on Obamacare while Sinema’s no vote directly harmed Americans struggling to get by on low wages.

At the time, Sinema said she was protecting the Senate as an institution – that she supports raising the wage but wanted to back the ruling of the Senate parliamentarian, who had determined that the issue could not be included as part of the budget reconciliation process, which allowed the coronavirus stimulus bill to pass with 51 votes.

By sticking by the filibuster, Sinema could claim to be protecting the procedure for times when Democrats inevitably return to the minority, while avoiding the wrath of moderate Republicans in her state who were key to her win over McSally.

In recent days, however, the political winds have shifted as Biden has added his voice to those casting the filibuster as racist. During Biden’s first formal press conference at the end of March, he agreed with former President Obama that the legislative maneuver is a “relic of the Jim Crow era,” but suggested his immediate focus would be trying to prevent it from being “abused.”

At the same news conference, Biden labeled a new Georgia voting bill as worse than the “Jim Crow” laws that kept segregation and racist policies in place before the civil rights area. Georgia Gov. Brian Kemp countered that it actually expands voting opportunities in the state and simply implemented safeguards that Democrats fiercely oppose, such as requiring voter ID for mail-in ballots. Late last week, amid calls for businesses to oppose the new law, Major League Baseball pulled its All-Star Game out of Atlanta and Georgia-based Delta Airlines and Coca-Cola blasted the statute.

White House Press Secretary Jen Psaki on Monday said Biden has not called on the MLB or corporations to punish Georgia over the law, even though the president told ESPN that he strongly supports the decision to yank the All-Star Game from Atlanta.

"It's not something we're calling for from the White House,” she told reporters. “Our focus is on working with Congress to put in place voting rights legislation."

Manchin is the lone Democratic holdout not co-sponsoring S. 1, the Democrats’ sweeping national voting rights bill. Sinema has signed on to the measure as a co-sponsor even as she continues to back the filibuster – the rule that stands in the way of its passage.

Sinema’s attempt to straddle the two issues is beginning to face serious headwinds. A recent headline over an Arizona Republic opinion piece labeled her a “super villain.”

Conversely, state Republicans are equally adamant that she resist the pressure or face voters’ wrath. In their letter to Sinema, they cited “poll after poll” showing that the American people do not support efforts to weaken or eliminate the filibuster, including a Hill-HarrisX survey conducted Jan. 28-Feb. 1. That poll found that 52% of registered voters view the filibuster as an important tool for promoting compromise while 47% said it is primarily used to ensure gridlock.

“You, senator, are the last line of defense for the preservation of the dignity of American governance in the United States Senate,” the Republicans wrote.

Susan Crabtree is RealClearPolitics' White House/national political correspondent.