By John Whitesides

WASHINGTON (Reuters) - Another week, another Republican presidential debate.

The candidates competing for the Republican nomination to take on President Barack Obama go at it again on Tuesday night in their eighth televised debate -- the fifth since early September.

But this year's encounters have been more than bouts of name-calling and accusations. They have helped drive some of the campaign's biggest stories, from the surges of Michele Bachmann and Herman Cain to the punctured hopes of Rick Perry.

The debates also have drawn bigger television audiences than at a similar stage of the 2008 race, with a shifting field of candidates and a fired-up Republican base sparking heavy interest in the hunt for a challenger to Obama.

"The debates have defined the race more than usual this year," Republican strategist Ron Bonjean said. "They have been really successful at flushing out the strengths and weaknesses of various candidates."

At least three more debates are scheduled before voters get to have their say in a Republican race featuring stark momentum shifts for Bachmann and Perry and a string of big names like former Alaska Governor Sarah Palin who refused to run.

The unsettled field, the intensity of grass-roots Republican anti-Obama fervor and the schisms between Tea Party fiscal conservatives and the Republican establishment have helped fuel interest in the debates.

That interest also has been aided by the 24-hour cable news cycle and explosion of YouTube and comedy shows that feature clips from the debates around-the-clock.

"There is an echo effect after each debate, with all of these different clips being shown and viewed repeatedly," said Alan Schroeder of Northeastern University in Boston, author of a book on the history of presidential debates.

The impact was evident early. Bachmann's strong debut in a New Hampshire debate in June propelled the U.S. congresswoman to a short-lived surge in the polls and a win in the Iowa straw poll in August, but she ultimately drifted back in the pack.

'SELF-IMMOLATION'

Perry, the Texas governor, entered the race in mid-August and roared to the top, but a series of halting debate performances in September sent him tumbling back down.

Perry's deer-in-the-headlights answer to a question about the Taliban, and his fumbled attempt at accusing front-runner Mitt Romney of flip-flopping, might have been as damaging as the criticism from rivals about his policies on immigration or his order that young girls be vaccinated for a sexually transmitted disease.

"Perry's self-immolation has been almost completely a debate-driven phenomenon," Schroeder said. "And much of Cain's appeal has come from his debate performances."

Cain, the former pizza executive with the forceful speaking style, has used the debates to popularize his "999" plan to rewrite the tax laws and has taken off in polls.

Romney, the former Massachusetts governor who lost a 2008 bid for the White House, has been a steady and smooth debate performer who has delivered few highlights but also no gaffes or missteps in the first seven encounters.

Perhaps not coincidentally, he also has remained steady at or near the top of the pack in polls.

The Republican debate last month in Orlando, Florida, was the most watched so far this year, attracting more than 6 million viewers. That surpassed the biggest primary debate audience of 2007 -- 4 million for a Democratic debate that included Obama and rival Hillary Clinton.

All of the Republican debates shown on widely distributed cable television networks this year have drawn at least 3 million viewers.

"Republicans are very driven to beat Obama this year and they want to check out the field," Bonjean said. "People thought there would be too many debates, but it turns out they have been helpful."

(Editing by Alistair Bell)