Ohio Secretary of State Frank LaRose on Monday formally entered the state's Republican primary to take on Democratic Sen. Sherrod Brown next year.

"It's official: I'm running," LaRose said on Twitter. "I'm on a mission to give back to the state that has given me so much. To continue to serve the country I love and fight to protect the values we share. That's why I'm running to serve as your next United States senator."

The Buckeye State, which backed former President Donald Trump in the 2016 and 2020 elections, has become increasingly conservative over the past decade. Brown, a progressive with a populist streak, is vying for a fourth term but is considered one of the cycle's most vulnerable incumbents.

Ohio Republicans are now preparing for an expensive and potentially nasty primary, much like the contest in 2022 that ultimately sent J.D. Vance to the Senate, ahead of an even more costly general election campaign. Two unsuccessful candidates from that 2022 primary -- state Sen. Matt Dolan, whose family owns the Cleveland Guardians, and Cleveland businessman Bernie Moreno -- announced challenges to Brown earlier this year. Both are sitting on vast sums of personal wealth, while LaRose is expected to be a prolific fundraiser.

LaRose, who is currently serving a second term as Ohio's top elections officer, is a decorated Iraq War veteran and previously spent eight years in the state Senate. After narrowly winning the secretary of state office in 2018, he was reelected last year by 20 points.

LaRose is a leading proponent of a ballot measure that Ohioans will consider in August, which would make it more difficult to amend the state constitution.

The proposal, which LaRose had initially described as being in the interests of "good government," would raise the threshold needed to change the constitution in a statewide referendum from a simple majority to 60%. It also would require backers of ballot initiatives to get signatures from voters in all of Ohio's 88 counties -- rather than the current 44 -- to place something on the ballot.

But LaRose later said publicly what many had assumed -- that the measure was designed to push back against another potential ballot measure, which would enshrine abortion rights in the state's constitution.

Abortion rights groups submitted 710,000 signatures to LaRose's office on July 5 -- roughly 300,000 more than state law requires -- in a bid to get the measure on the November ballot.

"Some people say, 'This is all about abortion.' Well, you know what, I'm pro-life; I think many of you are as well, right?" LaRose said in May of the effort to raise the threshold. "This is 100% about keeping a radical, pro-abortion amendment out of our constitution. The left wants to jam it in there this coming November."

The effort to raise the threshold for an amendment and the likely ensuing fight over abortion rights will be expensive and fiercely contested. Democrats are hoping those fights mobilize their base ahead of the 2024 election, especially as all three leading GOP Senate primary candidates celebrated the US Supreme Court's decision overturning Roe v. Wade in 2022.

LaRose and Dolan, though, could face some headwinds among Republicans over their hesitance to embrace Trump. Dolan did not seek the former president's endorsement during his 2022 Senate campaign. LaRose said at a closed-door event reported on by Politico in May that the influence of Trump's support had diminished. He estimated that as much as 60% of the party "doesn't care who he endorses." Still, LaRose -- who before the 2020 election called it "irresponsible" for Republicans to claim without evidence that an election was "stolen" -- touted Trump's endorsement of his own reelection campaign ahead of last year's midterms. He also tweeted, about two months before the endorsement landed, that "President Trump is right to say voter fraud is a serious problem."

In March, LaRose announced that Ohio had left a voting integrity group targeted by Trump and conservative groups, joining several other Republican-led states. LaRose criticized the Electronic Registration Information Center, or ERIC -- a multistate consortium that aims to help maintain accurate voter rolls -- for refusing "to embrace reforms that would bolster confidence in its performance." Less than two weeks earlier, Trump had baselessly said that ERIC "pumps the (voter) rolls" for Democrats.

Despite LaRose's more recent actions, Moreno is more closely allied with the former president, who touted the Colombian immigrant's entry into the race this spring.

"Word is that Bernie Moreno, the highly respected businessman from the GREAT STATE of OHIO, and the father-in-law of fantastic young Congressman, Max Miller, is thinking of running for the Senate," Trump wrote in a post to his Truth Social site. "He would not be easy to beat, especially against Brown, one of the worst in the Senate!"

LaRose has not endorsed in the 2024 presidential primary but has committed to supporting the eventual GOP nominee.

This story has been updated with additional details.