WASHINGTON (AFP) - United States Senator John Kerry offered some tough love on Wednesday to the colleagues he leaves behind to become secretary of state: end the fierce partisanship or risk the nation's reputation as a beacon of democracy.

Mr Kerry, whose confirmation as President Barack Obama's choice to replace Mrs Hillary Clinton as chief US diplomat sailed through the Senate a day earlier, said lawmakers needed to end the chamber's "reputation as a sanctuary of gridlock" and get down to legislating through compromise and consensus.

"If democracy deadlocks here, we raise doubts about democracy everywhere," Mr Kerry, who at times turned emotional in his farewell speech on the Senate floor, told Democrats and a smattering of Republicans. "If we use the time to posture politically in Washington, we weaken our position across the world."

Lawmakers in the House and Senate have engaged in bruising partisan debate and voting in recent months, notably concluding the 112th Congress a month ago by waiting until the final hour to reach only a partial deal to avoid a series of fiscal crises including tax hikes and spending cuts.

Mr Kerry, a 28-year veteran of the chamber, appeared to be offering his words of advice in part as a plea for cooperation as he takes on the task of leading US foreign policy.

"As I prepare to represent our nation in capitals around the world, I am more than conscious that my credibility as a diplomat and ours as a country is determined to a great degree by what happens right here in our capital city," Mr Kerry said.

"The antidote to the current narrative of American decline" being put forward in rival nations like China and Iran, he said, "is to demonstrate that we can get our economic house in order - because we can be no stronger abroad than we are here at home. It's that simple."

Despite the harsh assessment, Mr Kerry spoke warmly of his colleagues across the aisle, particularly Republican John McCain, who, like Mr Kerry, fought in the Vietnam War but found "common ground" with him on several issues.

And the future secretary of state could not help but make light of how he, like Mr McCain, came up short in his presidential ambitions.

"Eight years ago I admit that I had a very different plan... to leave the Senate, but 61 million Americans voted that they wanted me to stay here with you," he said to laughter in the chamber.

Mr Kerry choked up several times during his 45-minute speech, including when he spoke of President John F. Kennedy and brother Ted Kennedy, from whom Kerry inherited his mahogany desk in the Senate when the younger Kennedy died in 2009.

Massachusetts Governor Deval Patrick on Wednesday appointed his former chief of staff, Mr William "Mo" Cowan, to temporarily fill Mr Kerry's Senate seat until a special election is held on June 25.

In picking a relative political unknown, Mr Patrick passed over other potential appointees including long-serving congressman Barney Frank, who retired early this month, and Mrs Vicki Kennedy, the widow of former US senator Ted Kennedy.

The departure of Kerry, whose confirmation breezed through the Senate on Tuesday, sets off a scramble for a seat in a liberal state where Democrat Elizabeth Warren defeated incumbent Senate Republican Scott Brown in November.