Washington (AFP) - Mitch McConnell, the perpetually unruffled US Senate majority leader, stood before a gathering of Christian conservatives recently and compared himself to a cemetery groundskeeper: "Everybody's under you but nobody's listening."

The quip earned guffaws from the crowd. But while leading the senators in President Donald Trump's party has been likened to herding cats, McConnell's impact is no joke.

On Tuesday, the 76-year-old Kentuckian becomes the longest serving Republican Senate leader ever, surpassing Bob Dole's 11-plus years in charge.

It is a remarkable run for a lawmaker with one of the lowest approval ratings in the United States, and all the more impressive given the tumult in the GOP since Trump took the reins.

But McConnell's power -- especially behind the scenes -- has been undeniable.

"He's good at it," Republican Senator Johnny Isakson tells AFP about his friend's role as leader.

"He's got an even temper, doesn't shoot from the hip, he's thoughtful and very smart."

McConnell lacks Trump's telegenic charisma and bravado. But rivals who misunderstand McConnell's soft-spoken manner for weakness do so at their peril.

While House Republicans will soon be electing their third leader in as many years, McConnell is on his third US president as Senate GOP leader.

"I'm proud of the fact that I've enjoyed the confidence of my members," McConnell told Politico recently, adding he hopes to run for re-election in 2020.

He is a top tactician, a quality sealed in his 2016 masterstroke, when he denied Trump's Democratic predecessor Barack Obama the chance to fill the Supreme Court seat held by Antonin Scalia, the conservative justice who died in February 2016.

Holding that seat open was key in galvanizing Trump's base and helping him win the election.

It was "the single most consequential decision that I've made in my entire political career," McConnell told the conservative gathering Friday.

McConnell, who first came to Congress in 1985, has spent a long career navigating and changing the system from within.

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"He understands the modern game of politics," said veteran Senator Richard Shelby.

"He keeps things close to his vest. That's why he's survived this long."

- 'Our champion' -

Through his 11-plus years as leader, from 2007 to 2015 as minority leader and the last three and a half in the majority, McConnell has not faced a serious challenge from within.

It is a tough daily grind in the Senate, where McConnell's party holds a slim 51-49 majority.

Sometimes, he has fallen short, as with Republican efforts to repeal Obamacare last year, and he has bristled at some of Trump's provocative moves.

But when it came time to be a loyal footsoldier, he led the charge, pushing massive tax cuts across the finish line.

Where the leader is cementing Trump's legacy most clearly is in the judiciary, with the Senate confirming large numbers of conservative federal judges.

"He has been our champion in getting judges confirmed," White House legislative affairs director Marc Short told AFP.

McConnell's savviest political move this year may have been to scrap most of the Senate's August recess, citing Democrats' "historic obstruction" of Trump nominees.

But the extra workdays in Washington will also keep Democrats who face tough re-election bids this year off the trail during a crucial campaign month.