Former U.S. Sen. Mark Udall is endorsing Jason Crow’s bid to unseat incumbent U.S. Rep. Mike Coffman in eastern metro Denver’s 6th Congressional District.

“To effectively serve, you need courage of conviction, passion for making your community a better place, and the support of your friends and neighbors,” the Democratic Party elder  says in a fund-raising e-mail for Crow going out on Thursday. “Jason truly fits the mold of a passionate public servant. But his path to victory is going to be an uphill climb.”

Udall asks supporters to donate at least $5 to Crow’s campaign and cites the candidate’s experience as a combat veteran and leadership style.

“I’m an avid mountain climber,” Udall writes. “Mountain climbing is challenging but rewarding – a lot like public service”

A former U.S. House member, Udall, of course, lost his Senate seat to then-U.S. Rep. Cory Gardner in 2014.

“It’s an incredible honor to have Senator Udall’s endorsement,” Crow said. “Mark Udall is the epitome of public service and what it means to put Colorado families first. It’s clear that’s the kind of leadership we’re missing in Washington from Mike Coffman, who over and over again has put his own career above the needs of Colorado’s communities.”

Locking up such a key endorsement, however, is a major step for Crow. He faces a growing field in the Democratic primary, including Aurora attorney David Aarestad and Littleton resident Gabriel McArthur. Tech entrepreneur Levi Tillemann also is likely to enter the race.

State Sen. Rhonda Fields is often cited as a contender for the Democratic nomination. Fields, a high-profile anti-crime advocate and former state House member, was elected to the state Senate last November. She hasn’t yet said how seriously she’s considering a challenge to Coffman, which would not require her to give up her spot as a state senator to run.

Coffman has proven very difficult for Democrats in what’s viewed each election cycle as winnable for the left because of the districts mix of moderate Republicans, independents and immigrants.

Democrats usually struggle in mid-term elections, but the party is counting on President Trump’s unpopularity next year. They counted on the same thing last year when Coffman beat former state Senate President Morgan Carroll handily.

“The same Colorado Democrats that strong-armed Bernie Sanders out of the nomination are back up to their old antics,” said Tyler Sandberg, a spokesman for Coffman’s campaign. “The Democratic establishment has been openly disdainful of the progressive grassroots for a very long time, so put it this way – no one is surprised the Democratic establishment is trying to crush Levi, Gabe and David.”