A new United States Congress convenes in Washington, DC, on January 3. But for the first time in 18 years, a key Republican leader will no longer be at the helm: Senator Mitch McConnell.

Since 2007, McConnell has served as head of the Republican Party in the Senate, steering members of his caucus through four different presidencies and countless legislative hurdles.

Experts say his tenure as the Senate’s longest-serving party leader will ultimately be remembered as an inflexion point for Republicans and Congress as a whole.

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Upon vacating his leadership post in January, McConnell is expected to take on the role of chairman of the Senate Appropriations Subcommittee on Defense.

In his new position, he is likely to advocate for strengthening the US military to counter threats from adversaries like Russia, Iran and China.

Yet at age 82, with health challenges including a recent fall, experts say McConnell is unlikely to offer much resistance to the incoming Trump administration.

“Since Senator McConnell is out of his leadership position and given his physical frailty, I am not expecting much in the way of sustained opposition from him,” Harvard University political scientist Daniel Ziblatt told Al Jazeera.

“It is possible he may cast a dissenting vote here or there that might make a difference. But his track record doesn’t leave me holding my breath.”