Bennet, who was appointed when President Barack Obama named Ken Salazar interior secretary, led the race by 47.5 percent to 47.1 percent, The Denver Post reported.
Bennet had an estimated 7,000 votes more than Buck, with 30,000 ballots still to be counted in Boulder County. A recount would be triggered if the vote difference amounts to less than one-half of 1 percent, or 3,900 votes, the Post said.
Democratic Denver Mayor John Hickenlooper won the governor's race with 50.4 percent of the vote to 36.9 percent for third-party candidate Tom Tancredo, who represented the American Constitution Party, and 11.2 percent for the Republican nominee, political novice Dan Maes.
"The global recession didn't start here, but we intend to stop it here," said Hickenlooper. "Our focus was on the issues ... Our problems are too big for partisan politics."
In the state's House races, Democrat Diana Degette took the 1st Congressional District from Republican Mike Fallon. In the 2nd District, Democratic candidate Jared Polic edged out Republican Stephen Bailey.
In the 3rd Congressional District, Tea Party favorite Republican Scott Tipton beat Democratic opponent John Salazar. In the 4th Congressional District, Republican challenger Cory Gardner beat Democratic Rep. Betsy Markey.