It might still be more than 20 months away, but the battle for Arizona's Senate seat in 2024 may be the hottest race to watch.

When Sen. Kyrsten Sinema announced in December she would step away from being a Democrat and reregister as an independent, it opened the door for plenty of speculation about whether any Democrats would step up and run in a potential three-way race.

Those questions were answered on Monday, when Rep. Ruben Gallego announced he would challenge Sinema. That prompted a host of stories, like one from Slate, that asked how such a race would work, and whether or not Sinema would even run again.

Part of Sinema's challenge is her popularity in the state. A Morning Consult poll from early in January found the party switch made her slightly more unpopular in the state, with 50% of voters disapproving of her and 38% approving.

Sinema wasn't particularly popular with Arizona Democrats before the switch, with 41% disapproving of her job, but that jumped to 59% disapproval. The move did help her with independents in the state, where she's running even with disapproval approval ratings, and Republicans, where the plurality still disapprove of her job.

While Sinema has some friends among Democrats in the Senate, where she still caucuses with the party, GOP Sen. Minority Whip John Thune, R-S.D., was reported in The Hill as encouraging Sinema to begin caucusing with Republicans to avoid a three-way race in 2024.

A three-way race could open the door for Republicans, depending on who they nominate. One survey that included recent GOP gubernatorial candidate Kari Lake in a race with Sinema and Gallego had Lake at 36%, Gallego at 32% and Sinema at 14%, according to Newsweek.

Gallego's early entrance means a head start on the campaign. In fact, he set a one-day record in donations, collecting more than $1 million within 24 hours of his announcement from more than 27,000 individual donations. That bested the record set by Sen. Mark Kelly, according to CNBC.