It's Election Day in Los Angeles, where voters will pick their next mayor after a long and expensive campaign. The technically nonpartisan runoff election pits two Democratic political insiders against one another: City Councilman Eric Garcetti and City Controller Wendy Greuel.

The race has attracted the attention of organized labor, Bill Clinton, and a longtime ally to President Obama. Here's a rundown of the biggest things to keep in mind about the race and why Tuesday's election matters well beyond the city limits:

1) Can Clinton's candidate spring an upset? And how much do endorsements matter?

The former president endorsed Greuel about three weeks after she advanced to the runoff. A former Clinton administration Housing and Urban Development hand, as well as an early backer of Hillary Clinton's 2008 presidential campaign, Greuel has deep connections to the former first family. Bill Clinton has stumped for Greuel, and a super PAC supporting her highlighted his endorsement in a TV ad.

Greuel is trailing Garcetti by seven percentage points, the most recent public poll showed. If she can come from behind and win, it will mean another Clinton ally in a high-level position. If Greuel falls short, the question of how much high-profile endorsements matter in certain campaigns will be resurrected. In addition to Clinton, Greuel has the support of House Minority Leader Nancy Pelosi, Sen. Barbara Boxer, and EMILY's List.

2) Will Garcetti's Obama connection matter?

(Kevork Djansezian/Getty Images)

Obama made a point to stay out of the race, but Garcetti has been playing up his ties to the president. Former top Obama strategist David Axelrod supports the councilman, who was an early Obama supporter in 2008. "I think it's fair to say that the President of the United States could not have a better friend than Eric Garcetti," Axelrod said on the stump earlier this month. Greuel slammed Garcetti's campaign over a flier showing Obama and other black leaders giving "support" to Garcetti.

 3) Will labor's big investment pay off or fall flat?

Garcetti holds a union sponsored campaign flier, which is being distributed in the poor Latino neighborhoods of Los Angeles telling voters that Greuel would raise the minimum wage to $15 per hour if elected mayor, standing next to a chart showing total contributions for both candidates during a news conference on May 13. (Kevork Djansezian/Getty Images)

Organized labor has been largely in Greuel's corner, and the groups have been shelling out a lot of cash to try to get her elected. Overall, outside spending has been very one-sided in favor of Greuel. Working Californians, a super PAC funded substantially by unions has raised more than $4 million to support her.

Garcetti has sought to tether his opponent to public unions, questioning whether she could stand up to them after enjoying such strong support. A Garcetti victory would no doubt be a blow to public labor organizations  considering how much money they poured into the race. But to be clear, he is a liberal Democrat who is not viewed as an enemy of labor. So we're not talking about a setback to unions on the scale of Wisconsin Gov. Scott Walker's recall win, or Michigan's transformation into a "right to work state," for example.

4) History will be made -- but how?

(AP Photo)

No matter who wins, history will be made in the City of Angels. Garcetti would be the first elected Jewish mayor, while Greuel would be the first woman to win the top job.

5) How low will turnout go?

(Jessica Kourkounis/Getty Images)

Turnout was very low back in March. It could set a record Tuesday -- and not in a good way. A Los Angeles Times analysis concludes the winner may win fewer votes than any newly elected mayor elected since the 1930s. The timing of the election could hardly be worse for attracting attention. It's an off-year election just before Memorial Day. From a candidate perspective, low turnout could be good news for Greuel, who is backed by a strong organizational apparatus anchored by labor groups.