The Senate seat of the late Sen. John McCain, who died in 2018, is up for grabs in the upcoming election. Because the newly elected senator will need to complete McCain’s term, this is considered a special election, and the winner will take their seat immediately in November, rather than in January like all other newly elected senators. Depending on which party wins, President Donald Trump‘s mission to appoint a new Supreme Court justice following the death of liberal Justice Ruth Bader Ginsburg on Friday could hang in the balance.

Due to the proximity of the November 3 election, there is a lot of debate on whether or not Trump should be allowed to nominate someone. Trump has stated that because the seat opened up while he was still in office, he has an “obligation” to nominate a new justice. He has vowed that this nominee would be a woman, out of respect for Ginsburg, and wants to name his nominee within the week.

The race in Arizona is between Democrat Mark Kelly, and Republican appointed Sen. Martha McSally. Kelly has held a consistent large polling lead over McSally. If he were to win, and be sworn in as soon as the results of the election are official, his seat would eliminate one Republican vote for Trump’s nominee. The Republicans currently hold 53 seats in the 100-member Senate; with this narrow a margin for votes, an extra Democrat vote could be fatal to Trump’s nominee. Two GOP senators have already said that the next president should make the nomination.

In the case of a tie, Vice President Mike Pence would break the tie in Trump’s favor.

McSally and Kelly differ in opinion over Trump’s desire to select a new justice. Shortly after Ginsburg’s death, McSally tweeted that “this U.S. Senate should vote on President Trump’s next nominee for the U.S. Supreme Court…if Mark Kelly comes out on top, HE could block President Trump’s Supreme Court Nominee from being confirmed.”

Kelly argued for a go-slow approach. “When it comes to making a lifetime appointment to the Supreme Court, Washington shouldn’t rush that process for political purposes,” Kelly said in a statement. “The people elected to the presidency and Senate in November should fill this vacancy.”

Because it is a special election, Arizona law requires election results to be certified on the fourth Monday after the election (November 30), the seat will be filled before the possible end of Trump’s presidency.