WASHINGTON (AP) — Arizona Democrat and former astronaut Mark Kelly was set to be sworn into the Senate on Wednesday, an ascension that will narrow GOP control of the chamber while underscoring his state's shift from red to blue.

Kelly, 56, defeated GOP Sen. Martha McSally in last month's election, making her one of only three incumbents to lose. His arrival will reduce the Republican edge in the chamber to 52-48.

That will have scant impact on Majority Leader Mitch McConnell's control over the chamber for the final month of this congressional session. But it sets the stage for two pivotal Jan. 5 Senate runoff elections in Georgia.

If Democrats win both, they will command the 50-50 chamber for the new Congress that begins in early January because Vice President-elect Kamala Harris would cast tie-breaking votes.

Kelly was expected to be sworn into office by Vice President Mike Pence. Among those planning to watch were his wife, former Rep. Gabby Giffords, D-Ariz., and Scott Kelly, Kelly's twin brother and fellow retired astronaut.

Kelly's taking office is a political milestone for Arizona, which narrowly backed Democratic President-elect Joe Biden last month and will have two Democratic senators for the first time since January 1953. That is when GOP Sen. Barry Goldwater took office, barely a decade before he became his party’s unsuccessful 1964 presidential candidate.

Democrat Kyrsten Sinema is Arizona's other senator.

McSally was appointed to her seat in 2019 to replace the late GOP Sen. John McCain. Her appointment lasted only until last month's special election was officially certified, which occurred this week. That cleared the way for Kelly to take office and fill the rest of McCain's six-year term, meaning Kelly will face reelection in 2022.

Kelly was parachuting into a fractious lame-duck session in which lawmakers and President Donald Trump are so far deadlocked over whether to provide a pre-holiday COVID-19 relief package worth hundreds of billions of dollars. They’re also trying to address year-end budget work and a defense policy bill.

Kelly cast himself as a problem-solving centrist during his campaign. His slender 2 percentage point victory over McSally suggests he will be part of Democrats’ moderate wing.

In what was one of the country's most expensive Senate races, Kelly raised $89 million. That was second only to the $108 million collected by defeated South Carolina Democratic Senate candidate Jaime Harrison, according to the nonpartisan Center for Responsive Politics.

Republican Cory Gardner of Colorado and Democrat Doug Jones of Alabama were the only other Senate incumbents defeated last month.

The son of two police officers, Kelly is a retired astronaut who flew four space missions, including spending time aboard the International Space Station. He was also a Navy pilot who flew combat missions during Operation Desert Storm in the early 1990s.

Giffords was grievously wounded in a 2011 mass shooting in which six people were killed and a dozen others hurt. She and Kelly became leading figures in unsuccessful efforts to pressure Congress to strengthen gun controls.

Kelly will be the fourth astronaut to be elected to Congress. John Glenn was a Democratic senator from Ohio and Harrison Schmitt was a GOP senator from New Mexico. Republican Jack Swigert was elected to the House from Colorado, but died of cancer before taking office.