MADISON, Wis. (AP) -- Something striking happened when Wisconsin voters went to the polls in 2004.

Tens of thousands of them backed Republican President Bush's re-election and on the same ballot voted to send one of his most liberal critics, Democratic Senator Russ Feingold, back to Washington.

These ticket-splitting voters made a huge difference. The Republican president came up just short in this politically divided state, losing to Democrat John Kerry by 11,000 votes. But Feingold trounced his GOP challenger by 331,000 votes.

Who these fiercely independent "Feingold voters" back in this year's presidential race could decide whether Republican John McCain or Democrat Barack Obama wins Wisconsin's 10 electoral votes.

Feingold is endorsing Obama and has heartily introduced him on campaign visits. But he's said in the past that his friend McCain would be a good president and will be tough to beat in Wisconsin.