A three-way race, coupled with the risk that Sinema and the eventual Democratic nominee will split the vote, would complicate the party's already uphill battle to maintain control of the Senate in 2024. Democrats will be forced to defend 23 seats, including Sinema's and two others held by independents, compared with just 10 seats for Republicans.
With tough and expensive races on the horizon, it remains unclear just how firmly the Democratic establishment and major donors will line up against Sinema, who has voted for most Democratic legislation even as she's stood in the way of major priorities for the White House, congressional leaders and the progressive movement.
“I'm assuming that they will be with us because we are going to run the winning campaign, and because at the end of the day, if you look at where Arizonans are going to be, they’re going to be with us and not with her," Gallego told the AP.
A spokesperson for Sinema, Hannah Hurley, declined to comment on Gallego's announcement.
Gallego, an acerbic presence on social media who is quick to take down rivals from both parties, floated the idea of challenging Sinema to raise money last year and has for weeks been publicly assembling a team of advisers, hiring Democratic campaign veterans with experience working on tough swing-state Senate races in Arizona, Georgia and Pennsylvania.
He announced his campaign with an online video that shows him talking to veterans at an American Legion post in Guadalupe, a Latino and Indigenous community just outside Phoenix. He said his path from humble roots, beating the odds by getting accepted to Harvard University, motivates him to fight to preserve the American dream, first in the military and now in politics.
The son of immigrants from Mexico and Colombia, Gallego was raised in Chicago by a single mother, after his father was imprisoned for dealing drugs. He enlisted in the U.S. Marine Reserves while he was on a break from Harvard, where he struggled with culture shock. He wrote in a 2021 book, "They Called Us Lucky," that he was asked to leave during his sophomore year, when he partied too much, his grades slipped and he broke unspecified rules. He was later allowed to return.
He fought in Iraq in 2005 in a unit that sustained heavy casualties, including the death of his best friend, and he struggled with post-traumatic stress disorder after returning. He moved to Arizona to join his Harvard girlfriend, who had become active in Democratic politics in the state. The couple married in 2010 and divorced in 2017, a month before their son was born. His ex-wife, Kate Gallego, is now the mayor of Phoenix.
Gallego was elected in 2010 to the state Legislature, where Sinema also served for one of his two terms. In 2014, he won a bitter congressional primary, toppling a dynastic figure in the Phoenix Latino community. He's giving up a safe Democratic seat in Congress, a district that includes the Black and Latino neighborhoods of south and west Phoenix.
In Congress, he has focused on veterans and military issues.
Sinema has modeled her political approach on the maverick style of the late Republican Sen. John McCain of Arizona, who alienated the grassroots of his party by sometimes crossing the aisle to work with Democrats. She's become a fierce advocate of bipartisan compromise in an era when extreme partisanship has made it much more difficult.
She has been at the center of many of the biggest congressional deals of Biden's presidency, from a big, bipartisan infrastructure package to a landmark bill to legally protect same-sex marriages. But she's also become estranged from many Democrats, who blame her for voting down progressive priorities like a minimum wage hike and watering down others, like Biden's big social spending initiatives. She single-handedly thwarted Democrats' longtime goal of raising taxes on wealthy investors.
Her support for maintaining the filibuster, a Senate rule requiring 60 of 100 votes to pass most legislation, has made her a pariah among Democrats, who need Republican support to pass most bills despite controlling a majority of seats. The tension reached a head in 2021, when Democrats tried and failed to pass voting rights legislation.
Sinema doubled down on her position last week, telling global elites in Davos, Switzerland, that democracy didn't collapse in the 2022 election despite her support for the filibuster.