Neither fund raising nor the building of grass-roots organizations in key primary states is driving the Republican presidential race. Endorsements haven’t mattered much either. Stump speeches have been of minimal importance. And policy papers—such as former Massachusetts Gov. Mitt Romney’s 59-point economic plan or ex-Utah Gov. Jon Huntsman’s proposal for tax rate cuts—have been largely overlooked.

By far the biggest influence on the Republican contest has been the series of nationally televised debates. There have been more debates than ever—six so far—and they have attracted record audiences. The most recent debate on Sept. 22 on Fox News drew more than six million TV viewers, plus another six million watching on streaming video.

The debates have overwhelmed the Republican race. “They are about all there’s been to the campaign,” says Fox political commentator Brit Hume. After each debate the campaign has been frozen until the next one, except for arguments over issues spawned by the debates themselves.

Gov. Rick Perry’s policy of offering instate tuition at Texas colleges for illegal immigrants, and his effort to require 12-year-old girls to be inoculated against HPV virus, became prominent issues once he was pelted with questions about them in the debates. When Mr. Romney attacked Mr. Perry’s position on Social Security, it emerged as a front-burner issue. Mr. Perry gave the issue a news hook by calling Social Security a “Ponzi scheme.”

While Mr. Romney’s record in Massachusetts was always going to be a major issue, the debates have made it more so. The same is the case with Mr. Perry’s opposition to erecting a fence across the U.S. border with Mexico.

Another effect of the debates has been to make the Republican race more combative than it might otherwise have been, at least this far ahead of election year. The media like conflict, encourage it, and have succeeded in generating it.

The impact of the debates on the candidates has been palpable. After three poor debate performances last month, Mr. Perry dropped out of first place in polls. He fell to 17% in the ABC News/Washington Post survey in late September, from 29%. He trailed Mr. Romney (25%) in that poll and was tied with businessman Herman Cain.

The reverse is true for Mr. Cain. His conservative message and personal appeal in debates have increased his support to 17% in the ABC News/Washington Post poll in late September, from 3% in late August.

Rep. Michele Bachmann of Minnesota has risen, then plummeted, chiefly because of debates. With a strong performance in the June 11 debate on CNN, she was included for the first time in the Rasmussen presidential preference poll, getting 19%. Less impressive in more recent debates, she’s slipped in the Rasmussen poll to 8%.

The rise and fall in poll numbers reflects the normal fickleness of voters in the early stages of a multicandidate presidential campaign. But the debates have reinforced the tendency to rotate from one candidate to another.

By focusing heavily on Mr. Romney and Mr. Perry, the three debates in September heightened the impression, if not the reality, of a two-man contest for the GOP nomination. Time after time, questioners raised an issue with one of them, then turned to the other for a response. Mr. Romney provoked spirited clashes by challenging Mr. Perry’s stances on Social Security, immigration and the Texas economy.

During the Sept. 7 MSNBC debate at the Reagan Library in Simi Valley, Calif., Mr. Perry said Democratic Gov. Michael Dukakis “created jobs three times faster than you did, Mitt.” Mr. Romney fired back: “Well, as a matter of fact, George Bush and his predecessor created jobs at a faster rate than you did, governor.” He was referring to Mr. Bush’s time at Texas governor from 1995 to 2001.

Mr. Romney asked Mr. Perry at the Sept. 12 CNN debate in Tampa, Fla., if he believed Social Security should be turned over to the states. “I think we ought to have a conversation,” Mr. Perry responded, trying to brush the issue aside. “We’re having that right now, governor,” Mr. Romney replied. “We’re running for president.”

The flurry of presidential debates is a cable television phenomenon that began after Fox and MSNBC were launched in 1996. The number of debates has increased each presidential cycle. This year’s seventh debate, sponsored by the Washington Post and Bloomberg Television, is scheduled for Tuesday.

Before cable stepped in, the few televised debates were held just before the early caucuses and primaries and occasionally later if the battle for the nomination was undecided. TV debates before the election year were rare.

Audiences have grown. The first debate, televised by Fox in May, was viewed by 3.3 million households. The next five: on CNN in June (3.2 million), Fox in August (5 million), and in September on MSNBC (5.4 million), CNN (3.6 million), and Fox (6.1 million).

The biggest beneficiaries of increased viewership are the also-rans. The debates keep them in the race. At no cost, they get the attention of millions more voters than they ever could on their own. And the debates put them on equal footing with candidates who are well-financed and better organized. The marginal candidates have no incentive to drop out.

Once the also-rans sign up for a debate, the stronger candidates are all but required to show up, if only to avoid the embarrassment of being represented before a national audience by an empty chair. The also-rans and the cable people have them over a barrel.

For Republicans, a campaign dominated by televised debates has two disadvantages. It puts the folks they loathe, the press, in control. The media can dwell on subjects such as tax cuts for the rich or social issues that one or more of the candidates would prefer not to discuss. They are hard pressed to squeeze their talking points into the dialogue. Mr. Obama gets a pass.

Since the media pit the candidates against each other, Mr. Obama’s strategists get an early glimpse of the vulnerabilities of the Republican candidates, their strengths and weaknesses on issues, and the attacks used most effectively against them. All Mr. Obama has to do is sit back and enjoy the show.

Mr. Barnes is executive editor of the Weekly Standard and a commentator on Fox News Channel.