What was a little fuzzy last month has become clearer of late: The sagging economy is weighing on Google (GOOG).

Goldman Sachs analyst James Mitchell cited signs of weakness in search advertising – Google’s biggest moneymaker by far — in cutting his revenue growth target for the current fourth quarter from 4% to 1%.

Mitchell is the second analyst this week to lower estimates for Google. On Monday Barclays analyst Doug Anmuth called for fourth-quarter sales to be flat with third quarter’s $4.05 billion. If so, it would be the first time in Google’s history that revenue has not grown from one quarter to the next.

The prospects of little or no growth sent Google shares to a three-year low of $300 Tuesday amid a broad market selloff. The stock has dropped 55% this year. And the news was not particularly good for the rest of the Internet sector with Yahoo (YHOO) dropping 5% to a 52-week low of $11.25.

Why the sudden chill? Google thermometer readers apparently saw a change in tone between the company’s discussions on the earnings call Oct. 16 and the comments in the quarterly filing on Nov.7.

In October, when asked on a conference call how search traffic patterns were going, CEO Eric Schmidt couldn’t draw any conclusions. “We see fluctuations and they are more complex than they may appear; some things go up, some things go down.”

By Friday, when Google filed its 10Q, the complexity had simplified in a not-so-positive way.

“The current general economic downturn will likely affect these seasonal trends, especially the increase in commercial queries that we typically experience in the fourth quarter of each year,” Google said in the filing.

In October, Google executives were partially blinded by the huge increase in search traffic as people swarmed online for news on the financial crisis after Lehman Brothers collapsed Sept. 14. But as search traffic fell back after the initial panic subsided, consumers didn’t revert to their normal behavior and spend their time searching topics like holiday electronics. Ominously for Google, the rate of click-throughs on the sponsored ads that appear to the right of search results also fell, according to some analysts. Marketers pay Google only when someone clicks on their ads.

Goldman’s Mitchell writes that a poor economy is leading to declines in paid search and he noted that the size of purchases at sites like eBay (EBAY) and Amazon (AMZN) are shrinking. Given the lack of pay off, advertisers are likely to resist price increases or even cut spending, according to Mitchell.

In other words: A not-so-happy holidays ahead for the online economy.