Biden spoke at a rally in Greeley on Wednesday, one day after the second presidential debate.
He says Romney was "sketchy" and gave few details on his plans. He also accuses the Republican presidential candidate of being in a "time warp" on women's issues.
Biden urged the crowd to vote early and noted that Colorado could decide the election.
Colorado is one of a handful of hotly contested swing states. Romney, President Barack Obama, their running mates and family members have made numerous visits to the state.
Biden plans to make a campaign stop in Nevada later Wednesday.
Obama appears to have 237 of the 270 electoral votes needed for victory comfortably in hand, and Romney is confident of 191. That leaves 110 electoral votes up for grabs in nine battleground states: Florida (29), Ohio (18), North Carolina (15), Virginia (13), Wisconsin (10), Colorado (9), Iowa (6), Nevada (6) and New Hampshire (4).
(KUSA-TV © 2012 Multimedia Holdings Corporation with The Associated Press)