The final unanswered question about the field for the coming United States presidential election has been resolved - with presumptive Democratic nominee Hillary Clinton choosing Virginia Senator Tim Kaine as her running mate.

So the November election will feature Mr Donald Trump and Indiana governor Mike Pence facing off against Mrs Clinton and Mr Kaine.

The former secretary of state announced her decision on Twitter and through SMS late on Friday. "I'm thrilled to tell you this first: I've chosen Senator Tim Kaine as my running mate. Welcome him to our team," she wrote.

Later, in a fund-raising e-mail to supporters, she praised the 58- year-old as a "lifelong fighter for progressive causes", adding that he was "one of the most qualified vice- presidential candidates in our nation's history".

And she stressed that her decision rested heavily on the question of whether he could step in to the role of president if needed.

"I have no doubt that Tim can do the job. I want him by my side on the trail and in the White House."

Mrs Clinton's announcement concludes a months-long, highly secretive process. Massachusetts Senator Elizabeth Warren was rumoured to be among those in the running.

Mr Kaine was, however, always considered the front runner and his selection was generally regarded as a safe choice for Mrs Clinton.

"He's not controversial, so he won't overshadow Hillary Clinton on the campaign trail. In addition, he has a wealth of political experience," said Dr Kyle Kopko of Elizabethtown College, author of a widely referenced book on vice-presidents called The VP Advantage.

Similarly, an analysis from the University of Virginia concluded Mr Kaine would not move the needle much: "He probably won't inspire many new converts to Team Clinton but he probably won't drive many away, and he seems unlikely to produce negative headlines..."

He has been a governor, a senator and even chairman of the Democratic National Committee in a political career that has seen his national profile steadily rise.

Throughout that two-decade career, the father of three has also developed a reputation as a nice guy, a rare commodity in the cut-and- thrust world of politics. And that reputation was evident when Mr Kaine received tributes even from some Republicans on Friday.

"Trying to count the ways I hate Tim Kaine. Drawing a blank," Republican Senator Jeff Flake wrote on Twitter. "Congrats to a good man and a good friend."

Mr Kaine, who passed two stringent vetting processes, was also among the contenders to be President Barack Obama's running mate in 2008.

Strategically, however, it is not clear how much Mr Kaine brings to the ticket. He speaks fluent Spanish, thanks to a year spent as a Christian missionary in Honduras, but will not have the same appeal among the Hispanic community as a V-P pick who is actually Hispanic.

He comes from a swing state, but one that Mrs Clinton is favoured to win, and is ideologically moderate, so unlikely to help with the more progressive wing of the party.

If anything, the criticism that has emerged focuses on how much of a centrist Mr Kaine is.

The senator voted in favour of giving President Obama fast-track authority to negotiate the Trans-Pacific Partnership last year, breaking from many members of his party. He has also not condemned the deal, unlike Mrs Clinton.

Some Republicans have sought to cast the Democratic ticket as one filled with political insiders.

"Ultimately, this is a ticket that represents one thing: four more years like the last eight, just with more corruption and scandal," said Republican Party chairman Reince Priebus.