“It definitely opens up the Senate for potentially a full spectrum of conservatives to win — or potentially lose the seat entirely,” said Representative Trent Franks, an Arizona Republican. “The possibilities are almost endless.”
Mike Noble, a Republican pollster, called it “bedlam out here in Arizona.”
“Everyone’s phones are blowing up,” he said. “This is essentially a free-for-all.”
For Democrats to narrow the Republican hold on the Senate, they will have to take aim at two Republican seats, Mr. Flake’s and Senator Dean Heller’s in Nevada. No other Republican up for re-election next year comes from a generally competitive state.
But while they have talked a big game in Arizona, hoping for a demographic and political shift, so far, Democrats have little to show for their efforts. The party does not hold a single statewide office. In 2012, the last time the state had an open Senate seat, Democrats recruited Richard H. Carmona, a tough-talking Vietnam War hero and former surgeon general in the administration of George W. Bush. Despite Mr. Carmona’s conservative credentials, Mr. Flake won the seat, 49 percent to 46 percent.
Credit Ross D. Franklin/Associated Press
Next year could be different. The challenge for Republican Party officials will be to find a candidate who is aligned closely enough with Mr. Trump to appeal to his populist wing of the party — and to beat Ms. Ward in a primary dominated by the hard right — but not so closely that he or she would lose to Ms. Sinema.
The party needs “an acceptable ‘not Kelli’ choice who can surf the policy divides that are besieging our party right now,” said Chuck Coughlin, a Republican strategist in Arizona. He and other strategists say they have been inundated with text messages and phone calls since Mr. Flake’s surprise decision to leave the race.
Mr. Flake, who lost support with Trump supporters in Arizona after publishing a book, “Conscience of a Conservative,” that took aim at the president, announced his retirement this past week with a blistering indictment of Mr. Trump and the Republicans who embrace him.
But while he framed his decision as a matter of principle, polling in the state showed just how difficult it would be for an anti-Trump Republican to win an Arizona primary. One recent poll showed Mr. Flake with an 18 percent approval rating, and a survey in August by Arizona High Ground, Mr. Coughlin’s firm, found Mr. Flake trailing both Ms. Ward and Ms. Sinema.
Ms. Ward has the backing of Stephen K. Bannon, Mr. Trump’s former chief strategist, who has vowed to wage war on establishment Republicans, chief among them Senator Mitch McConnell of Kentucky, the majority leader. Ed Rollins, a veteran strategist who is the co-chairman of Great America, a Trump-aligned political action committee, recently signed on as her campaign manager.
And Robert Mercer, the deep-pocketed New York investor who helped finance Mr. Trump’s campaign, has donated $300,000 to Ms. Ward’s political action committee this year, according to Federal Election Commission records.
But Steven Law, the chief executive of the Senate Leadership Fund, a political action committee aligned with Mr. McConnell, vowed that Mr. Bannon and his allies would be the losers.
“Senator Flake’s decision to exit the race gives everyone a chance — the White House and Republicans in Arizona — to work together to find a strong candidate who can hold this seat,” Mr. Law said in an email. “That’s not going to be Kelli Ward.”
Arizona House Republicans met privately in Washington on Thursday morning to discuss who might challenge Ms. Ward. At least one potential candidate, Representative David Schweikert, told reporters last week that he lacked “the burning passion” to run. No consensus was reached on an alternative.
Speculation has centered on State Treasurer Jeff DeWit, an early and enthusiastic Trump backer who was the chief operating officer of the president’s 2016 campaign, and Representative Martha McSally, a retired Air Force colonel and the first American woman to fly in combat.
Credit Bob Christie/Associated Press
But it is unclear whether Mr. DeWit is interested. And with Mr. McCain battling brain cancer, party operatives say Ms. McSally — who has long expressed a desire to serve in the Senate — may be betting that his seat will open up, and that she could ascend to the Senate by appointment of the governor.
Ms. McSally has also drawn opposition from a coalition of conservative advocacy groups, led by the Club for Growth. On Friday, the Club for Growth released a letter urging Matt Salmon, a former representative from Arizona, to jump into the race, calling Mr. Salmon “a proven, principled conservative.” Mr. Salmon, who is now the chief lobbyist for Arizona State University, is considering it, a person close to him said.
Whoever wins the Republican primary will almost certainly face Ms. Sinema, who is pitching herself to voters as a consensus builder with a compelling life story: She was homeless as a child, and her family lived for a time in an abandoned gas station.
In a brief interview while taking a break from fund-raising phone calls, the congresswoman described herself as “really committed to finding a bipartisan solution” to the nation’s problems, and said she subscribed to both liberal and conservative values — a philosophy she said had roots in her childhood.
“We got help from both family and my parents’ church, but we also got help from the government,” she said, adding that she believed in “the conservative concept of pulling yourself up by your bootstraps and working really hard every day,” and in the liberal notion that “you’ve got to help people in need.”
Ms. Sinema, 41, is also the first openly bisexual member of the House, and has the strong backing of gay rights groups, including the Human Rights Campaign, the powerful Washington advocacy organization, which has vowed to make “significant investments” in Arizona races this year.
But some of her positions have rankled party progressives. Besides her Benghazi vote, she also voted against Representative Nancy Pelosi of California for Democratic leader.
There is “still a lot of resentment” toward Ms. Sinema over her Benghazi vote, said Mr. Grijalva, who added that he would withhold his endorsement until he learned more about how she would vote on issues including tax cuts, immigration and the environment.
“Given how Kyrsten Sinema defines herself in the next few months, she could solidify her position with a real chance to win,” Mr. Grijalva said.
One potential challenger to Ms. Sinema is Mark Kelly, the retired astronaut and husband of former Representative Gabrielle Giffords, who was badly wounded in a 2011 massacre in Tucson. This past spring, some grass-roots Democratic groups in Arizona drafted a petition encouraging Mr. Kelly, who now advocates gun safety, to enter the race, and they are said to be renewing that effort.