Since 1976, a Democratic presidential candidate has won Colorado just once - Bill Clinton in 1992. Al Gore in 2000 and John Kerry in 2004 each lost to George W. Bush.
Both parties targeted Colorado in 2004, but Democrats pulled support for John Kerry before the election, and he lost the state by 100,000 votes, political consultant Eric Sondermann said.
"Ask Al Gore or John Kerry if they would have wanted nine electoral votes. Those nine electoral votes would have made Al Gore president," the bipartisan Sondermann said Monday. "I'm unaware of any historical precedent for Colorado being this much in play."
Colorado offers Republicans a better chance of success than other target states such as Pennsylvania, Ohio and Virginia, where Democrat Barack Obama has bigger leads in polls, political consultant Floyd Ciruli has said.
Today, the electorate is just about evenly divided among Republicans, Democrats and unaffiliated voters.
Voter turnout already has been intense with 1.6 million mail-in ballots and 365,000 absentee ballots turned in from the state's 2.6 million active registered voters. The statewide turnout is expected to set a record, exceeding 90 percent.
In the presidential race, Obama has led Republican John McCain by single digits in recent state polls.
And, the campaigning continues at a fever pitch.
In Colorado Springs, GOP vice-presidential candidate and Alaska Gov. Sarah Palin on Monday asked supporters to send presidential candidate John McCain on "one more mission," telling voters time is short.
"Election Day is just hours away. The time for choosing is near. Are you ready to send us to Washington to shake things up? Are you ready to send an American hero on one last mission?" she asked a packed hanger at the Colorado Springs airport a day before the election.
She was shaking hands after her speech when word came out that a report cleared her of ethics violations in the firing of her public safety commissioner. Campaign officials said they weren't surprised.
"It's exactly what we expected," said Tom Kise, McCain's Colorado campaign manager said.
Palin told the crowd Barack Obama is out of touch with small businesses and predicted his tax plan would be ruinous, taxing businesses at the expense of the economy.
In Littleton, Michelle Obama, her voice choking with emotion, asked supporters in Colorado on Monday to thank Barack Obama's grandmother for raising him.
Obama used her final Colorado campaign appearance - and her first since the Democratic campaign announced the death of the woman Barack Obama called "Toot" - to ask supporters to pray for Madelyn Payne Dunham, who died at age 86.
"Say a prayer for Toot and thank her for raising Barack Obama. I think she did an amazing job," Obama told about 2,500 people at a suburban Denver high school gym.
"This is an emotional time for us," Obama said. "We were hoping she'd hang in there, but she didn't. But she knew what was going on."
Barack Obama announced the news from the campaign trail in Charlotte, N.C.
The joint statement with his sister Maya Soetoro-Ng said Dunham died late Sunday night after a battle with cancer.