Vice presidential choices aren’t supposed to make much difference. Yet, Sarah Palin’s impact is undeniable and extremely large. Twelve days ago, presidential election futures markets indicated that Barack Obama had a 62 percent probability of winning the election. By Sunday, Obama’s probability had fallen to 46.5 percent, with John McCain at 52.1 percent.

With the election at stake and Palin so crucial to the outcome, Democrats have sent “a mini-army of 30 lawyers, investigators and opposition researchers” into Alaska. The media has also understandably descended in large numbers in Alaska. Despite Palin’s name being on the short list of potential vice presidential nominees for months, neither Obama’s campaign nor the media were prepared.

Yet, as Hillary Clinton’s own pollster has warned:

“... the media is on very dangerous ground. I think that when you see them going through every single expense report that Governor Palin ever filed, if they don't do that for all four of the candidates, they're on very dangerous ground. I think the media so far has been the biggest loser in this race. And they continue to have growing credibility problems.... I think that the media is doing the kinds of stories on Palin that they're not doing on the other candidates.”

The media, though, has done more than simply subject Palin to questions not given to the other candidates. More importantly, it has been running interference for the Obama campaign, questioning McCain’s attacks on Obama while frequently reinforcing Obama’s on McCain.

Take a couple of examples from last week. McCain ran an ad criticizing Obama’s support for sex education for kindergartners. Seventy percent of the 2,774 news stories on McCain’s ad mentioned that the ad was a “lie” or “inaccurate.” The stories’ headlines indicate that if other ways of disparaging the McCain ad were included, this percentage would probably be much higher.

Obama’s supporter defend the legislation, saying that its purpose was to protect children from sexual abuse. True, the bill proposed teaching children not to talk to strangers, but one wonders if reporters actually read the entire bill. For example, the legislation also included this: “Each class or course in comprehensive sex education offered in any of grades K through 12 shall include instruction on the prevention of sexually transmitted infections, including the prevention, transmission and spread of HIV.” How can you teach how to prevent these different sexually transmitted diseases without getting into some details about sex?

By the way, a grand total of 22 out of 2,774 stories that mention the bill include this discussion of HIV transmission, and many of those stories were still negative about McCain.

For a more direct comparison regarding Palin, look at how the media discussed the whole lipstick-on-a-pig story. The majority of media coverage again questioned the accuracy of the ad that the McCain campaign used to strike back at Obama (using words such as “inaccurate,” “false,” “misleading,” “untrue,” and “lie”). Obama says that he was surprised that anyone could think he was referring to Palin when he said: “you can put lipstick on a pig; it’s still a pig," that the direct reference was to Bush’s policies.

But there are a few problems with Obama’s response. First, his audience thought he was referring to Palin. They started laughing and shouting after he said, "You can put lipstick on a pig," before he even got to the punch line. The AP wrote: Obama's "audience, clearly drawing a connection to Palin's joke." ABC’s Jake Tapper wrote that audience members told “reporters that they thought Obama had been alluding to Alaska Gov. Sarah Palin’s” lipstick line. Politico noted: "Though on a day when Obama's surrogates were joking that Palin's record can't be concealed with lipstick, it was hard for those following the campaign not to hear the echo." The crowd also started chanting “No more pit bull,” indicating that the crowd got the reference to Palin’s quip at the Republican convention.

A Google news search found only 20 stories out of 6,074 mentioned the crowding laughing or shouting, and only two of those mentioned that they laughed or shouted before the punch line was given. Another search found only one single news story by US News & World Report that even mentioned both Obama’s lipstick joke and the crowd chanting “no more pit bull!”

Whatever Obama’s original intent, it would have been hard for him to claim that he was surprised that the McCain campaign would also draw the connection between his joke and Palin. Nor is it clear why he didn’t make a statement given the crowd’s reaction.

In Palin’s case, far from trying to protect her, the press frequently doesn’t even acknowledge that there might be arguments to be made in her favor.

-- The Bridge to Nowhere. The Alaska Democratic Party posted a Web page in which it declared that “State of Alaska killed bridge” -- that is the “Bridge to Nowhere.” However, after Palin’s vice presidential nomination the party took down the Web page. Yet, out of 7,556 news stories on the Bridge to Nowhere this month, a Google news search produces no hits on the phrase “State of Alaska killed bridge.”

Similarly, Sen. Ted Stevens, who was the king of the earmarks for Alaska and who had a “frosty” relationship with Palin because of her opposition to those earmarks, noted that “I don’t remember her ever campaigning for it. She was very critical of it at the time.” With all the news stories on this question, one would think that Stevens’ comment would be newsworthy, since it was his pork-barrel project. But there were only three news stories that mention his statements.

-- Cutting Earmarks. The media on this question has definitely been a bunch of “the glass is half empty” types. Instead of noting what Palin had accomplished, the game was identifying any earmarks that Alaska still received. Palin “cut nearly 10% of Alaska’s budget this year” and reduced the number of federal grants from 54 in 2007 to 31 in 2008 -- a $350 million, 64 percent cut in requests. Among the cuts in Alaska budget were a $30,000 van for Campfire USA and $200,000 for a tennis court irrigation system. Further cuts in future years were also promised.

But those changes weren’t the standard by which the media wanted to judge Palin. For example, Charles Gibson thought that he had gotten Palin in his interview when he noted that Alaska still got “$3.2 million for researching the genetics of harbor seals, money to study the mating habits of crabs” from the federal government.

Again, a simple Google news search shows how incredibly lopsided this coverage has gotten. From September 1 to September 15 there were 9,222 news stories that discuss Palin and earmarks. By contrast, a search that looked at Palin and her attempts to cut earmarks (using the terms "cut earmarks," "reduce earmarks," "trim earmarks," "slash earmarks" or "eliminate earmarks") produced incredibly only 50 news stories -- about 0.5 percent of the total stories. Even many of those 50 stories were critical and claimed that McCain had overstated Palin’s opposition to earmarks.

I spent some time looking at questions of whether Palin was properly vetted or the taxes that she imposed on oil companies, but the results were similar. Palin just doesn’t seem to get an even break, let alone the extremely protective news coverage offered Obama.

Groups such as FactCheck.org have helped put down many false rumors on everything from Palin supposedly cutting funds for special needs children to banning books, but the coverage that corrects these false claims never seems to be as heavy as the coverage making the claims to begin with.

Possibly there is a good explanation for why the media so selectively covers the two campaigns so differently. But whatever the reasons, Sarah Palin continues to receive significantly less positive coverage than the Democrats.

Lott is the author of Freedomnomics and a senior research scientist at the University of Maryland.

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