Updated

ST. LOUIS (AP) — Republican Missouri Attorney General Josh Hawley has made it official: He's running for the U.S. Senate against incumbent Democrat Claire McCaskill in 2018.

Hawley, a 37-year-old in his first year of elected office, will release a video Tuesday morning announcing his candidacy. His campaign made the video available to The Associated Press.

He said he wasn't planning to run for Senate. "But we believe we have to do all we can to win a better future for our country," Hawley said on the video.

"It's time to do something new," Hawley said. "America is an exceptional place, and it's still a young country. Its future is worth fighting for. So let's get ready and do our part."

McCaskill and Missouri Democrats have anticipated the announcement for months. The Missouri Democrat in July launched a digital ad accusing Hawley of using the attorney general's office as a stepping stone to higher office.

Hawley said on the video that he and his wife didn't initially plan a Senate run. "But we believe we have to do all we can to win a better future for our country," he said.

Timmy Teepell, a senior adviser to Hawley's campaign, said in a statement that Hawley will formally launch the campaign in 2018 and wait until then to make campaign appearances.

McCaskill, 64, is in her third term in the Senate, but Missouri voters have increasingly favored Republicans in recent years. Just one statewide office holder, Auditor Nicole Galloway, is a Democrat, and only two other federal lawmakers are Democrats — congressmen William Lacy Clay of St. Louis and Emanuel Cleaver of Kansas City.

The Senate campaign figures to be a mirror image of the 2016 campaign involving Missouri's other incumbent senator, Republican Roy Blunt. In that campaign, Blunt, first elected to Congress in 1996, held off a highly-regarded young Democratic challenger, Jason Kander.

Unlike Kander, though, Hawley seems to have momentum on his side. Once considered a swing state, Missouri has turned decidedly Republican in recent elections. In statewide races in 2016, Republicans replaced Democrats as governor, attorney general, secretary of state and treasurer, and Republican Donald Trump easily carried the state over Democrat Hillary Clinton. In fact, Republicans haven't lost a statewide race since McCaskill defeated Republican Todd Akin in 2012.

McCaskill is among 10 Senate Democrats running in states won by Trump, and the Missouri race could be crucial in control of the Senate, where the GOP currently has a narrow majority of 52 seats.

Former Sen. John Danforth of St. Louis has publicly announced his support for Hawley and encouraged him to run. But Hawley must also bring in the more conservative wing of the Republican Party, a balancing act that could prove difficult.

The Kansas City Star reported last week that Hawley spoke with former White House strategist Steve Bannon after Bannon helped Roy Moore win the Alabama Republican primary in September over incumbent Sen. Luther Strange. A spokesman for Hawley did not immediately respond to a request to confirm if Hawley was seeking the support of Bannon, who left his White House post in August and returned to Breitbart News, which he led before joining Trump's campaign.

Vice President Mike Pence in July called Hawley about the race. Prominent donor David Humphreys also is among high-profile Republicans who released a letter in April encouraging Hawley to run.