WASHINGTON — Senator Jeff Flake’s decision to abandon his 2018 re-election campaign in Arizona has thrown open the Senate race there, exposing deep fissures not only on the Republican side where a nationalist insurgency is gunning for the party establishment but also among Democrats contending with a rising left.

Establishment Democrats have high hopes for Representative Kyrsten Sinema, a centrist who recently dined in the Blue Room of the White House with President Trump, teaches a bipartisan spin class in the House gym and has broken ranks with her own party on key votes. She was one of only seven House Democrats, for example, who voted to create a select committee to investigate the 2012 attack on the United States diplomatic compound in Benghazi, Libya.

But those impulses toward bipartisanship have soured Arizona progressives against the candidacy of a woman who rose to prominence on a biography that included time as a homeless child and an identity that includes being the first open bisexual in Congress.

“There are issues, murmurs within grass-roots groups and the progressive community, the environmental community and others, including immigration advocates,” said Representative Raúl M. Grijalva, Democrat of Arizona and a co-chairman of the Congressional Progressive Caucus, who is withholding his endorsement of her.