The 2004 race for the White House has produced the most expensive U.S. presidential advertising campaign in history, tripling the amount spent four years ago.

• INDEPTH: U.S. election 2004

U.S. President George W. Bush and his challenger, Senator John Kerry, spent a record $600 million US on radio and television advertising.

George Bush and John Kerry (AP photos)

The Democrats were the biggest spenders, shelling out $250 million US since March. Bush and the Republicans spent $240 million US.

The breakdown is also reflected in spending by outside groups. Liberal organizations spent $70 million US on advertising, while conservative groups forked over about $40 million US.

Despite the record spending, the bulk of the advertising is only run in 17 states with particularly close races.

This year's spending got off to an early start as the Republicans launched a spring election ad against Kerry, and the Democrats responded.

Analysts believe 2002's campaign finance reform law also contributed to the increase in spending. The law bars political parties from collecting money from corporations and unions.

Parties instead raised money from wealthy individuals, who are subject to contribution caps.

The so-called "527" groups also raised millions for advertising. Named for a section of federal tax law, the interest groups were allowed to raise unlimited amounts of money to run ads in support of, or attacking, candidates.

The now infamous Swift Boat ads, which stated Kerry "cannot be trusted," were financed by a 527 group, Swift Vets and PoWs for Truth.