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DENVER — The surprise announcement Wednesday afternoon that Congressman Cory Gardner had decided to challenge Democratic U.S. Mark Udall after all felt like a political earthquake.

In a matter of moments, most of the other candidates in a weak primary field were stepping aside or making plans to.

Republicans, on the whole, were suddenly united in their enthusiasm and belief that Gardner finally presented them with an opportunity to end a decade-long losing streak when it comes to top statewide races in Colorado.

“This is the best news that Colorado Republicans have had in a decade,” gushed former GOP state lawmaker Rob Witwer on Wednesday afternoon.

Democrats, most of whom were a bit stunned, came to their senses quickly and acknowledged that Colorado just became home to a top-tier Senate race that’s going to draw millions of outside dollars into the state this fall and, chances are, boost Republicans up and down the ticket.

But don’t go feeling too sorry for them.

However deeply felt Wednesday’s tremors were, the ground has not yet shifted in the world of Colorado politics.

Gardner’s candidacy just means Democrats are going to have to work harder to hold on to Udall’s seat and their majority in the Senate.

And they’ll probably have to revamp their usual playbook.

In a sense, Udall was asking for it.

Last summer, the first-term Democrat was formidable enough that Gardner, a careful, risk-averse politician, wasn’t about to risk his future on a tough race.

But the problem-plagued roll-out of Obamacare has hurt Democrats across the board, and Udall has been no exception.

While his office’s emails to the state Dept. of Insurance questioning the number of policy cancellations didn’t look great, it might have been his embarrassing post-State of the Union interview on CNN that encapsulated his sudden insecurity.

Asked whether he’d want to campaign with President Obama this fall, Udall clammed up live on national TV. Failing to answer CNN reporter Dana Bash’s simple question — not once, but three times — he looked like a politician suddenly afraid of his own shadow (not one with almost $5 million in the bank and a field of potential opponents inspiring zero confidence from the National Republican Senatorial Committee or the super PACs plotting where to spend millions this fall in negative advertising).

After seeing that, why wouldn’t Gardner reconsider?

Already, Democrats have been taking shots at Gardner: over a voting record that’s extremely conservative — he supported the Personhood measure in 2010 — and a House of Cards-esque re-shuffling of the GOP Senate field Udall’s campaign has dubbed the “Centennial State Swap” (Ken Buck, who’d been the leading candidate to take on Udall, quickly stepped aside for Gardner and announced a run for his soon-to-be vacant House seat).

And all of those attacks will be dusted off and repeated over and over again from here to November.

But you can expect Gardner to have a one-word response: “Obamacare.”

Having told his own family’s story of getting a cancellation letter and finding its premiums to have doubled, Gardner has been among the Republicans beating the Obamacare drum the loudest since enrollment began in October.

“I think what happened to his family is a big reason he’s so passionate about this issue,” said Rep. Amy Stephens, one of the former GOP Senate candidates who ended her campaign — unlike Buck, she did so with no golden political parachute — after Gardner decided to run.

“I think that’s what made him decide to do this.”

Personal passions being what they are, it’s more likely Gardner’s late campaign is a matter of pure political calculation by Republicans who saw Udall as increasingly vulnerable and Gardner as a viable enough candidate to draw serious outside money into the race.

And those calculations seem logical enough.

What’s really changed is this: Democrats no longer have the luxury of playing this one on their own terms, by beating up a fatally flawed candidate well outside the mainstream of a majority of Colorado voters with attacks on social issues.

Gardner, a smoothly aggressive, message-disciplined machine, will still have to answer for his positions on social issues; he may even have to finally say how he voted last year on a ballot measure in Yuma County to secede from Colorado.

But he’ll be tougher for Democrats — and voters, suburban women and other independents — to dismiss as “too extreme.”

And, let’s be honest: Colorado’s seen that campaign playbook before and is likely to see through another clumsy effort to run the same cookie cutter campaign that Democrats used to take down Ken Buck in 2010 — a game plan that only worked because Buck played right into it — against every conservative candidate since.

“Cory Gardner isn’t going to give us the YouTube moments and gaffes that Ken Buck did,” said Craig Hughes, the Democratic strategist who managed Sen. Michael Bennet’s narrow 2010 win over Buck. “But, unlike Ken, he has a voting record; and it’s a very conservative one.

“Watching him over the last several years, it never seemed like he was voting in a way to set him up for a statewide race. It seemed like he was positioning for House leadership. That’s another reason why his announcement this week was surprising.”

Last year, should have taught Democrats the limitations of the so-called “Bennet model.”

When trying to defend two state lawmakers in recall elections that came about as part of a backlash over their support of gun control bills, Democrats tried to avoid talking about guns altogether through the short recall campaigns, attempting to focus the public on their entire body of work and on the virtually unknown Republican successor candidates’ support for — wait for it — Personhood.

Both Democrats lost their seats.

In November, they did the same thing on a major ballot measure to increase education funding, tiptoeing around the fact that Amendment 66 was a pretty big income tax hike.

Again, they chose not to defend the policy at the heart of the matter; and, again, they lost.

Democrats should be wary of approaching the Udall-Gardner race the same way.

Yes, they can still go after Gardner for his conservative record and pick apart all sorts of votes; and, yes, they can and will remind voters that he’s part of the same unpopular GOP majority that shutdown the government last year and has failed to move on immigration reform, something Gardner himself said would get done following the 2010 election.

But they will also have to make the case for Obamacare, not apologize for or run away from it, just as Vice President Joe Biden — can’t believe I’m saying this — urged Democrats to do on Thursday.

“We should not apologize for a single thing,” Biden told the Democratic National Committee. “We should go out and flatly lay out each of the races. … This is who we are, this is who we stand for, this is what we’re going to do.”

Because it’s guaranteed that Gardner will be saying that one word more than any other this fall.

And it’s what Udall says in return that will like determine the outcome of what just became a heavyweight fight.