Senate Minority Leader Mitch McConnell (R-Ky.) is urging his fellow Republicans to buck up Sen. Kyrsten Sinema of Arizona — a Democrat, sources familiar with the conversations tell Axios.

Why it matters: Republicans view Sinema and her moderate Democratic colleague Sen. Joe Manchin of West Virginia as their last line of defense against sweeping progressive laws — ranging from a $3.5 trillion social welfare bill to potentially irreversible structural changes like eliminating the filibuster and adding new states to the union.

Between the lines: Sen. Thom Tillis (R-N.C.) published an uncommon opinion piece Wednesday on the NBC News website thanking and crediting Sinema.

• He praised the bipartisan infrastructure deal and said she'd taken more political risk than anyone, given that many progressives are now campaigning against her because she's working with Republicans.
• "It's rare to have an elected official from one party publicly praising another from the opposite party," Tillis wrote, "but I'm doing just that because it's so essential for the future of our nation that Sinema holds fast in keeping the filibuster intact."
• "The importance of her effort," he added, "has been made clear as the Senate has worked to pass bipartisan infrastructure legislation rather than pushing through every liberal dream item."

Daniel Keylin, a senior adviser to Tillis, told Axios that McConnell's private encouragement had no bearing on the senator's decision to write his piece.

• "It was Senator Tillis' idea and had been in the works for many weeks," Keylin said. "It was motivated by her good-faith leadership of the infrastructure negotiations and other bipartisan working groups they are in."

The big picture: McConnell is urging support for Sinema in private conversations, telling those with relationships with her that now is the time to speak up on her behalf.

• He's also used his own media appearances during the past month to send public signals to his colleagues.
• During a June 23 appearance on Fox News Radio's Guy Benson Show, the minority leader said he admired Sinema and Manchin for their "courage in defending the Senate as an institution."
• And on Wednesday, McConnell told Fox Business host Larry Kudlow that Sinema was "very courageous" for saying she can't support a Democratic reconciliation package costing $3.5 trillion.

What they're saying: A spokesperson for McConnell declined comment.