The Service Employees International Union has endorsed Sen. Michael Bennet (D-CO) for reelection, giving Bennet a boost of support heading into the March 16 precinct caucuses, the winner of which is listed first on the primary ballot in August.

(Full disclosure: Bennet is the brother of James Bennet, editor of The Atlantic.)

Colorado is an unlikely state for a progressive-friendly senator to come from, and it's not the most pro-labor state in the union by any means, but Bennet has proven to be a significant ally: he supports comprehensive immigration reform, and his letter supporting a public option through reconciliation has come to signify the leading movement to keep the public option alive.

Bennet is considered vulnerable, but his race rates behind Indiana, Arkansas, Delaware, and Nevada as threats to Democrats' edge in the Senate. He currently trails Republican challenger Ken Buck by a slim margin.

The appointed senator is facing a primary challenge from former House Speaker Andrew Romanoff. Some think Romanoff poses a legitimate threat, though Bennet has the support of President Obama, who traveled to Colorado this month for two fundraisers on Bennet's behalf.

Bennet polls better than Romanoff against various GOP challengers, meaning a Bennet win in the August primary would give Democrats a better shot at retaining his seat.

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