Take a look at this old article in the New York Times from the primaries last year.

Of the $33 million Mr. Obama raised in the second quarter, about a third consisted of donations of less than $200 -- more than the $10 million raised in $2,300 checks from big donors. Mrs. Clinton, in contrast, raised $2.3 million in donations of less than $200. Contributions of $2,300 made up $12.3 million -- or more than half -- of the $21.5 million that she raised for the primary during the second quarter. Both candidates now have about the same amount of cash to spend on the primary.

Mr. Obama's roster of 258,000 donors has exceeded the national mailing list that Mrs. Clinton accumulated through her two Senate races and Bill Clinton's two runs for the White House. None of the other primary candidates in either party has claimed more than 100,000 individual donors.

In some ways, Mr. Obama's donor base differed from those of his rivals. An analysis of his Federal Election Commission filing using census data by ZIP code found that in the 3,210 ZIP codes with the largest proportion of black residents -- at least 25 percent -- Mr. Obama led the other candidates for both parties in money raised in the first half of the year. He received $5.2 million from those ZIP codes in the first half of the year, while Mrs. Clinton received about $3 million.

For a more recent take, see this article by the AP from May.

Riggs and Schwartz are foot soldiers in Barack Obama's 1.5-million-strong army of campaign contributors. Dozens of Associated Press interviews with donors and an AP financial analysis show how contributions that make only a soft ka-ching by themselves, arriving in increments of $10, $15 and $50, have collectively swelled into a financial roar that has helped propel Obama toward the Democratic presidential nomination.

Altogether, Obama's campaign has taken in an unprecedented $226 million, most of it contributed online. His donor base is larger than the one the Democratic National Committee had for the 2000 election.

These are hardly political fat cats. Ninety percent of his donors give $100 or less, and 41 percent have given $25 or less, according to the Obama campaign. Overall, he has raised 45 percent of his money in small contributions. Hillary Rodham Clinton's figure is 30 percent, Republican John McCain's is 23 percent.

Obama does have big donors, and they do have an influence on the campaign, as shown by this article in the Washington Post from April.

But those with wealth and power also have played a critical role in creating Obama's record-breaking fundraising machine, and their generosity has earned them a prominent voice in shaping his campaign. Seventy-nine "bundlers," five of them billionaires, have tapped their personal networks to raise at least $200,000 each. They have helped the campaign recruit more than 27,000 donors to write checks for $2,300, the maximum allowed. Donors who have given more than $200 account for about half of Obama's total haul, which stands at nearly $240 million.

Obama's success in assembling bundlers offers another perspective on a campaign that promotes itself as a grass-roots effort. While the senator from Illinois has had unprecedented success generating small donations, many made online, the work of bundlers first signaled the seriousness of his candidacy a year ago and will be crucial as he heads into the final Democratic primaries with a lead against Sen. Hillary Rodham Clinton (N.Y.).

The bundler list also sheds light on those who might seek to influence an Obama White House. It includes traditional Democratic givers -- Hollywood, trial lawyers and Wall Street -- and newcomers such as young hedge fund executives, Silicon Valley entrepreneurs, Chicago-based developers and members of the black business elite. One-third had never contributed to a presidential campaign, much less raised money.

The list includes partners from 18 top law firms, 21 Wall Street executives and power brokers from Fortune 500 companies. California is the top source, with 19 bundlers. Both Illinois and Washington, D.C., have six, and five hail from New York.

However, even this article notes that

The campaign maintains that its fundraising success among average Americans has lessened its reliance on big donors. Donations of less than $200 account for nearly half of Obama's contributions, compared with a third of Clinton's and a quarter of Sen. John McCain's, according to the Campaign Finance Institute. More than 1 million people have given money to Obama's campaign.

When the third quarter filings are posted, we'll be able to see better how this trend has fared during the general election campaign.

It's harder to see the influence of this internet-mediated small-donor revolution on down-ticket races, but a quick glance at the Daily Kos Orange to Blue page says something.  $150,000 in a House race means quite a bit.