Sen. John Thune (R-S.D.), the number two Republican in the Senate, is endorsing Kari Lake in her Arizona Senate bid, Axios has learned.

Why it matters: The Lake endorsement is a reversal from Thune's long-term criticism of the MAGA movement, as he seeks to be the next Senate GOP leader.

• Thune has a strained past with the Trump wing of the party, not endorsing the former president until late last month. Instead, he first threw his support behind Sen. Tim Scott's (R-S.C.) White House bid.
• Trump endorsed Thune's GOP primary challenger in 2022.
• Lake, who aggressively pushed theories that the 2020 election was stolen from Trump, is a darling of the GOP's right wing.

The big picture: Thune announced Monday that he was seeking to be the next Senate GOP leader.

• But Trump has been urging Sen. Steve Daines (R-Mont.) to run for the position, Axios reported last week.
• Lake and other Republicans who may be elected to the Senate would have a say over who will be the next GOP leader.

Between the lines: Lake is endorsed by Trump and a number of his allies, like Daines and Sen. John Barrasso (R-Wyo.), who is running for GOP whip.

• But she has also picked up the support of Thune and other lawmakers who are not as closely aligned with the former president, like Sen. Joni Ernst (R-Iowa).

What they're saying: Thune told Axios that the Arizona race is "critical for securing a Republican majority" in the chamber.

• "Kari Lake is the candidate in Arizona who will work to get the economy back on track and lower the cost of living for families, secure the border and enforce the law, and bring safety to our streets," Thune said.
• Lake said Thune "worked to confirm President Trump's judges and pass his agenda."
• "I am looking forward to doing that when we win Arizona and secure the Senate Majority," she told Axios.

The latest: Thune and 16 other Republican senators will host Lake for a fundraiser in D.C. on Wednesday, according to an invite of the event obtained by Axios.

• Other attendees include possible GOP leader hopefuls Daines and Sen. John Cornyn (R-Texas).