But a pack of Republicans and even a fellow Democrat are gaining on Sen. Michael Bennet.

Bennet is expected to report raising more than $1 million in the last fiscal quarter. He was appointed earlier this year to fill the seat of current Interior Secretary Ken Salazar. Bennet has raised more than $3.6 million since taking office.

Bennet's brisk fundraising pace puts him ahead of his many challengers. But the race has gotten more complicated since the last campaign disclosure.

Democrat Andrew Romanoff of Denver plans to report more than $250,000. And on the Republican side, former Lt. Gov. Jane Norton will report more than $550,000 raised last quarter.