California, long accustomed to being an ATM for presidential candidates looking to raise money, could play an important role in establishing the Democratic nominee when voters here cast their ballots on Feb. 5.

Yesterday, in fact, all three major Democratic candidates took a break from campaigning in neighboring Nevada, which holds its caucuses on Saturday, to make a pit stop in the Golden State.

Sen. Barack Obama (D-Ill.) spoke with single mothers in San Francisco and held a fundraiser in Atherton. At the same time, Sen. Hillary Clinton (D-N.Y.) swung through Southern California, talking about the economy in Santa Barbara and race issues in Compton. Former North Carolina Sen. John Edwards was in Los Angeles.

Yesterday’s visits are probably not the last. With 18 days until Super Tuesday, in an unusually fluid race, campaigns are targeting the nation’s most populous state by sending candidates, opening field offices and buying airtime.

An Obama office opened Sunday on El Camino Real in Palo Alto, next to Happy Donuts. For Monday, the Clinton campaign has planned a rally to open an office on San Antonio Road, also in Palo Alto.

Twenty-one other states vote on Feb. 5 in the Democratic race. The size of the electorate renders retail campaigning strategies, used in Iowa and New Hampshire, unfeasible.

“The appearances are going to be dwarfed later on by a series of advertising buys,” said Communication Prof. Shanto Iyengar, who teaches campaign analysis. “This state is simply too big for people to make personal appearances and win on that model. There’s no time.”

The flurry of activity creates openings for more Stanford students to get involved. Assuming the race will stay competitive through the first Tuesday in February, the extra attention also seems likely to increase turnout, analysts say, especially among passionate younger voters who are upset about the direction the country has taken under President George W. Bush and excited about the prospect of change.

In a Los Angeles Times poll released this week, Clinton leads Obama by 16 points among likely California Democratic voters. She has deeper support from women, Latinos — a critical voting bloc in the primary — and leading Democratic politicians around the state, including San Francisco Mayor Gavin Newsom, Assembly Speaker Fabian Nunez and Los Angeles Mayor Antonio Villaraigosa.

“Senator Clinton has a better organizational network going for her, in terms of endorsements and on the ground,” said Ed Constantini, a Political Science Professor who studies California politics and presidential nomination contests at the UC-Davis — where former President Bill Clinton campaigned Tuesday night (see adjoining story).

The legislature, with the support of Gov. Arnold Schwarzenegger, last year moved the date of the primary up from June so that California would have a greater influence in the battle for the White House. Since it’s nearly impossible to imagine the Republican candidate winning the state’s 53 electoral votes in November, it seems safe to say that there will be little direct campaigning in California during the general election.

The Democrats allow independents to vote in their primary here, but the Republican Party blocks non-registered Republicans from voting in its California primary.

Bill Whalen, a fellow at the Hoover Institution who specializes in California politics, guessed this would help Obama, who tends to perform better among younger, more independent voters not as tied into the Democratic Party establishment. It will likely hurt Sen. John McCain (R-Ariz.), he said, who has strong appeal to moderate and crossover voters.

High turnout could benefit Obama, according to Constantini, who has not taken sides in the race.

Another factor to watch is absentee ballots, available for voters to send in since Jan. 7. Whalen estimates that 40 to 50 percent of all voters could mail in their choices before Election Day. That might counteract momentum built up by a candidate who scores decisive wins in Nevada on Saturday or the South Carolina Democratic primary on Jan. 26.

Clinton and Obama will likely win their respective home states on Feb. 5, which makes California more important as a potential decider.

Delegates to the Democratic National Convention are divided up proportionally so Obama could still come out strong if he loses the popular vote statewide. An unexpected strong showing by Edwards, running with 10 percent in this week’s LA Times poll of California voters, could alter the dynamic.