[Update: A commenter with a longer historical perspective than mine says President Lyndon B. Johnson drew a crowd of 125,000 to a campaign rally. Still, 25,000 is the largest crowd in recent history, at least.]
There does not seem to be a reliable source to cite for this claim, but Sen. Barack Obama’s rally today in Des Moines might be the largest single-candidate political rally in Iowa history.

Sen. Barack Obama rallies a crowd estimated at 25,000 in Des Moines
Four years ago, at an evening rally at the Iowa State Fairgrounds, Sen. John Kerry drew about 15,000. At the time, many asserted that it was the largest crowd in the state’s long history of campaign rallies.
Today’s turnout for Obama was estimated by Des Moines police at about 25,000.
That number pales in comparison to the crowds Obama has drawn elsewhere — 100,000 in Denver, for instance — but by Iowa standards, crowds do not get much bigger.
GOP vice presidential nominee Sarah Palin drew a crowd of about 10,000 in Des Moines last weekend.