In Colorado’s last six statewide elections, voters have chosen the Democratic candidate, including the last two presidential elections.

Now Republicans across the nation view Colorado as one of perhaps six locations that could tip the balance of power in the U.S. Senate, and Democrats in this purple state are becoming increasingly uncomfortable.

The race under the microscope is incumbent Sen. Mark Udall (D-Colo.) vs. Rep. Corey Gardner (R-Colo.), and Udall’s camp acknowledges the most recent polls show Gardner with a one- or even two-percentage point lead. It stands as a disconcerting turn of events for all those Democrats in Colorado and across the nation who have such a vested interested in this race.

Only four months ago, the election was set up as a likely slam-dunk for Udall, who expected to face a far less challenging (and baggage-ridden) Ken Buck, the Weld County district attorney who unsuccessfully challenged Sen. Michael Bennet (D-Colo.) in 2010.

Surprisingly and with scant explanation, Gardner opted to abandon his own slam-dunk re-election efforts for his seat in the House, announcing that he would instead begin campaigning against Udall. Fellow Republican Buck stepped aside, and now stands a good chance of inheriting Gardner’s seat in Congress.

Essentially, the Republicans had chosen their most electable candidate to go against Udall in this most critical of races.

So far, it seems to be paying off, no matter how hard the Udall camp tries to paint Gardner as a pro-personhood, anti-immigration, environment-be-damned candidate – issues that seem to poll in Udall’s favor in most of Colorado, and certainly in his home town of Boulder. (Gardner wants everyone to remember, via his television commercials, that he grew up in rural Yuma and lives in the same house that his great-grandparents owned.)

The latter issue of oil-and-gas exploration/production/delivery has occupied some of the candidates’ focus (most specifically in the form of the Keystone Pipeline, which Gardner wants and Udall doesn’t), and Gardner’s camp described Udall as hypocritical in his rejection of Keystone at the same time that he supports exports on liquid natural gas.

But the real back-and-forth has come in the form of personhood and immigration press releases. For example, from a press release dated June 20, Udall describes Gardner’s “disturbing record on women’s access to healthcare.” And on June 26, Udall shifted gears with a release announcing that Gardner had voted “to allow immigrants to be blackmailed and threatened with deportation.”