Democratic Congresswoman Kyrsten Sinema of Phoenix is planning a 2018 run for the U.S. Senate seat held by first-term Mesa Republican Jeff Flake, according to several sources familiar with Sinema's plans.

At the same time, Phoenix Mayor Greg Stanton, another Democrat who's been undecided about his political future, is telling supporters he's preparing to run for Sinema's vacant seat next fall, according to sources familiar with Stanton's calls.

Sinema issued this statement Friday:

"I’ve heard from many Arizonans encouraging me to run for the United States Senate. It is something I am seriously considering. When I make any decisions, Arizonans will be the first to know."

Stanton could not be reached for comment.

Sinema and Stanton are expected to formally announce their plans in the next few weeks.

Both moves would be long-awaited next steps by two of the leading Democratic officeholders in Arizona.

Sinema, a three-term congresswoman who's nurtured a reputation as an independent, was the subject of "will she or won't she" speculation regarding a run for the Senate in 2016, when Sen. John McCain was up for re-election, and again this year.

Sinema has locked down her swing district and proved to be a prolific fund-raiser, with $3.2 million in cash on hand as of June 30, a solid base for a Senate run in Arizona.

Sinema would face civil rights attorney and political newcomer Deedra Abboud in the Democratic primary.

Flake is viewed as one of the two most vulnerable Republican senators in 2018.

Recent polling shows Flake with high disapproval ratings among Arizona voters, driven down in part by his recent votes for the Obamacare repeal.

In a book released last week, Flake torched President Donald Trump as "unpredictable" and "volatile," questioning why fellow Republicans remain loyal to him. Flake was Trump's staunchest Republican critic on Capitol Hill during the presidential campaign.

ALSO: Flake says Trump not unfit despite torching him in new book

Trump hasn't formally endorsed a Flake opponent, but in recent days Trump allies have thrown their financial and organizational support behind Kelli Ward, a former state legislator who lost to McCain in the 2016 Republican primary.

Stanton, meanwhile, has been weighing his next political move as he nears the end of his second and final term as mayor in 2019.

Whatever he does has been contingent on Sinema's plans.