Clinton has drawn impressive crowds too. They include 10,000 people in San Diego and another 10,000 in San Jose, shortly before she carried California on Feb. 5.
On Tuesday night, as Obama was sweeping the Virginia, Maryland and District of Columbia primaries, 12,000 people came to see Clinton in El Paso, Texas. Obama's crowd that night in Madison, Wis., was more than 17,000, with some turned away.
When the two Democrats campaign in proximity, Obama's crowds usually overwhelm hers. More than 17,000 people packed Seattle's Key Arena to hear Obama on Feb. 8, while 3,000 others were shut out. The same day in Tacoma, Wash., a Clinton rally drew about 6,000. Her audience in Seattle the night before was 5,000.
On Feb. 9, about 7,000 people came to see Obama in Bangor, Maine, (although only 5,700 could fit in the gym). In the nearby college town of Orono, Clinton was drawing about 2,000.
Republican John McCain, who regularly draws crowds of 1,000 to 1,200, has noticed Obama's drawing power.
''I would remind you, and I don't mean to diminish his success, but Howard Dean used to get really big turnouts as well,'' McCain told reporters last week, referring to the Democrat who flamed out after causing excitement in the 2004 race. ''If I had a crowd like that, I'd be thrilled. I congratulate him for attracting that number of people.''
Republican Mike Huckabee generally gets smaller crowds, around 500 to 700.
A crucial question is whether big political crowds point to big voter turnouts. Obama hopes so, and recent trends encourage his camp.
Democrats have swarmed to the early primaries, often outnumbering Republicans in regions where the opposite usually happens. Democrats exceeded Republicans in same-day primaries held this year in GOP-leaning New Hampshire, Oklahoma and Tennessee — all won by Clinton — and in South Carolina, Georgia and Louisiana, won by Obama.
In the Jan. 26 South Carolina Democratic primary, Obama clobbered Clinton and John Edwards, winning more votes than were cast for all the Democrats combined in the contested 2004 primary. In Missouri, a battleground state narrowly won by Obama this month, more than 820,000 Democrats voted, compared with about 585,000 Republicans.
It is not simply Obama's crowd numbers that dazzle, but also the fervor.
Many people, women in particular, are excited about the prospects of Clinton becoming the first female president. But many of Obama's listeners — black, white, young, old — seem almost in awe, and they often talk of being part of history.
Some shout ''I love you'' as he talks. A few quietly weep. When he takes the stage, the roar is often deafening.
Seattle Times columnist Danny Westneat covered both candidates and tried to describe the difference. In a Feb. 9 article he said Clinton ''was impressive'' at her Seattle rally, ''with a speech packed with policy specifics and a certain intangible steeliness that signals she's got what it takes to be president.''
But ''the Obamapalooza,'' he wrote, ''is a once-in-a-lifetime spectacle.''
The Bangor Daily News reported that in the past 20 years only a rock group ''and illustrious Maine basketball players'' have filled the Bangor Auditorium as Obama did Saturday. ''The audience gave riotous applause and pounded their seats when Obama mentioned his initial opposition to the war in Iraq,'' it reported.
Clinton and Obama regularly hold round-table talks and other small events between their rallies, and those sometimes generate valuable local news coverage. But it is the big-arena shows that draw the most attention.
Does that mean he is pulling away from Clinton?
''We don't know,'' said Kathleen Hall Jamieson, an expert on political communication at the University of Pennsylvania. In 1964, she said, Republican Barry Goldwater thought he would beat President Johnson because he often drew bigger crowds.
''Sometimes a higher percentage of your base turns out to see you because they're highly motivated,'' Jamieson said, which helps explain Goldwater's landslide loss. The danger for Clinton, she said, is if journalists conclude that Obama's big crowds indicate a universal voter preference, which could become ''a self-fulfilling prophecy'' through misleading reporting.
Perhaps Clinton will shock Obama in Wisconsin next week, or turn the tide later in Ohio, Texas or Pennsylvania. For now, however, Obama's huge crowds are intoxicating some politicians, such as Rep. Elijah Cummings, D-Md.
''This is not a campaign for the presidency of the United States,'' he shouted while introducing Obama in Baltimore last Monday. ''This is a movement to change the world.''