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Colorado Democrat Mark Udall's bid for a second term has become the most unexpectedly competitive U.S. Senate race in the nation this year — and for unexpected reasons.

Yes, Udall, 63, like other vulnerable Democrats, is already being pummeled by big-money conservative groups for his support of President Obama's health care legislation.

And, yes, even with a last name that carries historic weight in the American West, the low-key senator has been hobbled by incumbency at a time of historic dissatisfaction with Congress, and with party leader Obama's approval rating well south of 50 percent.

Udall's fortunes, however, appear much more lashed to a pair of developments back home:

-- Recent and at-times virulent voter backlash against the political agenda — from stricter gun laws to civil unions — pursued by progressives who, with Democrat John Hickenlooper already in the governor's office, took full control of the state Legislature two years ago.

-- The emergence of a strong Republican candidate, Congressman Cory Gardner, 39, whose decision to run forced from the field Tea Party favorite Ken Buck, the party's losing 2010 Senate candidate. Buck opted instead to run for Gardner's House seat.

Gardner's late-February announcement for the seat was hailed by state Republicans as a "game-changer," one that Rob Witwer, who served in the state Legislature with Gardner, says could help his party post up better against a superior Democratic voter data and get-out-the-vote infrastructure.

"The conservative infrastructure has not yet matched that of the progressives here, but it's getting there," says Witwer, who documented in a revealing book with reporter Adam Schrager how Democrats by 2008 came to dominate Colorado politics.

(A key Democratic player in the effort that upended the rock-solid Republican control of the state was multimillionaire Jared Polis, elected to Udall's congressional seat in 2008.)

The scant public polling that exists suggests the race, crucial to whether Democrats retain control of the Senate come fall, is a tossup; the political race analysts at the Cook Political Report and the Rothenberg Political Report still have the race leaning in Udall's favor.

Democrats nationwide are continuing to struggle for a narrative to motivate voters to get to the polls in this coming midterm election; that poses a challenge for Udall in a state where his fellow Democratic Sen. Michael Bennet barely eked out a victory over Buck in 2010.

And that was before state Democrats, by almost all accounts, overreached in a state that Obama twice won but which is still decidedly divided.

"Last year was an incredibly aggressive liberal year here," says nonpartisan Colorado pollster Floyd Cirulli. "Civil unions. Gun control. Environmental legislation. Labor legislation."

"It just poured out," he said. "It certainly hurt the governor, and it hurt the overall Democratic brand here that had been shaped by Hickenlooper, [who was] a moderate, business type."

Udall and Bennet both became identified, fairly or not, says Cirulli, with being part of an "aggressive liberal party, even though both are mainstream, center-left guys."

The response came quickly. Eleven rural counties had ballot initiatives last fall to secede from the state; five were approved. Voters in a recall election in September ousted two Democratic legislators who voted to strengthen gun control measures in a state where two of the nation's worst mass shootings — Aurora and Columbine — have occurred; a third legislator facing a potential recall resigned.

Colorado voters also rejected a billion-dollar school-funding ballot initiative heavily supported by Hickenlooper, teachers' unions as well as former New York Mayor Michael Bloomberg.

Witwer says he views the defeat of the education initiative, and its accompanying tax increase, as perhaps the most significant measure of voter backlash.

"It went down, by a lot," he says. "It was a large vote against a tax increase, and, at the same time, conservatives were sweeping onto school boards."

"It was a significant indication of a changing tide in Colorado," he says.

Cirulli, the pollster, predicts that voter turnout this fall will be about 30 percent less than it was two years ago, when Obama won the state and Democrats took over the Legislature. Republicans are more united behind Gardner, who is a capable fundraiser, and whose personality, like Udall's, has most frequently been described as "affable."

He's not at war with Republican Party leadership in Washington, and has discarded some of his more controversial positions — including one that would give "personhood" status to a fertilized human egg.

Gardner's campaign reported raising $1.4 million in the first three months of the year — most after his announcement.

Udall, who had about $4.7 million on hand at the end of last year, reported raising an additional $2 million-plus this year through March.

A strong supporter of environmental laws, Udall has also burnished his profile in recent years as one of Capitol Hill's most persistent critics of the National Security Agency's data collection practices.

"He's a strong incumbent," Witwer says, "and he's not prone to saying offensive things or making grand mistakes. His family name is well-known out West.

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