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DENVER — Colorado Sen. Mark Udall raised $1.5 million in the first three months of 2013, his campaign announced Monday.

That brings Udall’s cash on hand for next year’s reelection bid to around $2.5 million.

In the last fundraising quarter, only five other incumbent senators facing reelection next year posted higher figures than Udall, a Democrat from Eldorado Springs seeking a second six-year term in Congress.

According to Michael Sozan, Udall’s Chief of Staff who is currently serving as his unofficial campaign manager, the donations come from a lot of small donors and from some notable members of the Denver business community, including some prominent Republicans.

No Republican has declared that they’ll challenge Udall next year, making him the only swing-state incumbent without an opponent (although Udall’s seat rarely is listed among the top 10 competitive senate races next year).

Some of the more oft-mentioned names — former Congressman Bob Beauprez, former Lt. Gov. Jane Norton and Yuma Congressman Cory Gardner — have yet to hint that they’re all that interested in challenging Udall, which could open up the field for a lesser-known candidate from within the state legislature.