WASHINGTON – Sen. Michael Bennet of Colorado, considered among the most vulnerable Democratic senators up for re-election next year, said Tuesday he raised more than $2 million in the first quarter of the year.

He had more than $2.9 million in his campaign account as of April 1, according to his campaign. Eighty percent of his donations during the three months were of $100 or less.

Candidates are required to file their first-quarter campaign finance reports by April 15, but many release their numbers in advance.

"Michael's proud of the support he's received from grassroots donors across Colorado, and that's reflected by our fundraising this quarter," said his campaign manager, Dan McNally.

Bennet was appointed to the Senate in January 2009 to replace Ken Salazar after Salazar was named secretary of the Interior.

He won his first full term the next year, defeating Republican Ken Buck by fewer than 30,000 votes out of more than 1.67 million cast. The candidates and outside groups spent about $45 million on the race.

No Republican has announced in the 2016 contest for Bennet's Senate seat, yet.

In the entire 2010 campaign, Bennet raised $11.5 million. The amount Bennet took in during the first quarter of 2015 is likely to be the second most he has raised in a quarter — exceeded only by the $2.3 million his campaign brought in during the third quarter of 2010.

He is certainly going to need the money. The cost of Senate elections has continued to escalate, with more than $100 million spent on last year's contest in which Republican Cory Gardner unseated Democratic Sen. Mark Udall.

El Paso County Commissioner Darryl Glenn and Fountain resident Charlie Ehler have filed as a Republican candidates for the seat.

The GOP has already begun campaigning against Bennet, launching a series of automatic telephone calls criticizing him for voting to block consideration of a human sex trafficking bill in the Senate. Bennet and other Democrats voted against the legislation because it contained an anti-abortion provision.

The Cook Political Report rates Bennet's seat "leaning Democrat," and the second-most vulnerable Democratic Senate seat in the 2016 election after the one retiring Nevada Sen. Harry Reid is giving up.

The Rothenberg & Gonzales Political Report, which uses a slightly different rating system, has the Colorado Senate race in essentially the same position.

A sign of the national significance of the race is that Democratic Sens. Cory Booker of New Jersey and Claire McCaskill of Missouri already have sent out fundraising appeals on Bennet's behalf.

Bennet's campaign fund ended up $10,000 lighter after he decided to give that amount to charity to offset a donation he received in 2010 from a committee associated with Sen. Robert Menendez of New Jersey. Menendez was indicted last week on corruption charges.