Colorado Senate race down to the wire

Greenwood Village, Colorado (CNN) - In one of the tightest Senate races in the country, incumbent Democratic Senator Michael Bennet is working down to the wire to get voters to the polls. He spent part of the day in Boulder, where he encouraged college students to vote. He's ending his day in Denver.

Going into Election Day, he said he expected the race could come down to a few hundred votes. Democrats are counting on their grassroots efforts to make the difference for them.

Republicans are voicing more optimism because of the number of their party members voting. Ken Buck, a conservative backed by the Tea Party, spent part of his day attending "get out the vote" rallies in Douglas and Arapahoe counties. But his spokesman says he also spent part of the afternoon preparing for his speech tonight. Dick Wadhams, the state's Republican chairman, is expressing "cautious optimism" Buck will unseat Bennet. Buck said Monday he believes he'll have a comfortable lead despite polls showing a dead heat heading into Election Day.

Buck has been taking aim at Bennet as a Washington insider, too closely tied with President Obama's policies, particularly healthcare reform. Buck has said he favors a repeal of healthcare reform legislation. Bennet has been portraying his opponent as too extreme. Buck has said the science behind global warming is unproven. He also opposes abortion, including in cases of rape and incest.

The race is so contentious, it has the distinction of drawing the most money from outside groups in advertisements. The Sunlight Foundation, a non-profit group tracking outside money spent on races estimates $32 million dollars spent on Colorado's Senate race.

Independents are expected to be the deciding factor in this race. They make up one-third of the voters in Colorado. They were seen as key in President Obama's victory in Colorado in 2008 when he won the state by more than 8 point.

Results in 3 counties are being closely watched tonight: Jefferson, Arapahoe in the Denver suburbs and Larimer County which includes Fort Collins. They are seen as swing counties and early results there could be telling.