Last month, I wrote about Senate Republicans' attempts to expand the national playing field in 2014  -- an effort designed to give them some margin of error in their attempt to take back control of the chamber. Here's a snippet of that riff:

At the moment, Senate Republicans have two major holes in their “expand the map” strategy: Colorado and Iowa. Colorado has moved toward Democrats in the past two presidential elections but remains a place where Republicans need to be able to compete. The GOP field is currently led by Ken Buck, who is best known for losing a very winnable race for Senate in 2010. In Iowa, the retirement of Sen. Tom Harkin (D) should have made for a sterling pickup opportunity for Republicans, but their field, at least at the moment, is a mishmash of sort-of-knowns and unknowns. Keep an eye on those two states over the next few months. (The filing deadline in Iowa is March 14; in Colorado, it’s March 31.) Adding one of those two — or maybe even Minnesota making it into that second tier — would further expand Republicans’ paths to a majority.

Today, the Denver Post reported that Rep. Cory Gardner, a rising star within the party who had previously turned down the chance to challenge Sen. Mark Udall in November, has reversed course and decided to run. Gardner, who was first elected to the eastern Colorado 4th district in 2010, represents a significant upgrade over the current crop of candidates in the GOP field.  Until today, Ken Buck, who ran unsuccessfully for the Senate in 2010 -- losing a very winnable race -- was seen as the likely nominee and a likely general election loser at the hands of Udall. (Buck's consultant told Roll Call that he will drop out of the Senate race and run for the now-open 4th district.)

FILE- In this Tuesday, Oct. 15, 2013, file photo, view of the U.S. Capitol building in Washington. The Treasury reports how much of a budget deficit the U.S. government ran in January 2014, on Wednesday, Feb. 12, 2014. (AP Photo/ Evan Vucci, File)

Now, while Gardner ran a very strong campaign to unseat Rep. Betsy Markey (D) in 2010, there's no evidence of how he will wear with a statewide audience -- particularly against a proven vote-getter like Udall. Still, there are signs of trouble in the air for Democrats in the state. A Quinnipiac poll conducted earlier this month showed Udall in a statistical dead heat with his no-name Republican challengers. That same poll showed that the electorate was evenly divided on whether Udall deserved to be re-elected or not, and that six in ten disapproved of how President Obama was handling his job.  Those numbers -- coupled with the fact that Colorado is widely regarded as one of a dozen or so swing states nationally -- make Gardner instantly competitive.

Democrats will assuredly disagree about Gardner's chances of ousting Udall but won't dispute the fact that Colorado just went from nowhere to competitive in the space of the last 24 hours.  Here's why that matters for the national fight for the Senate.

Republicans need to win six Senate seats to retake the majority. In West Virginia, South Dakota and Arkansas, Republicans recruited their strongest candidate and, at worst, have even odds to take those Democratic seats. That gets the GOP halfway to its goal. With the addition of Colorado to the map, Republicans are now aggressively targeting eight other seats: Alaska, Colorado, Louisiana, Michigan, Montana, New Hampshire, North Carolina and Virginia. While some of those seats are longer shots than others (Virginia, we are looking at you), all eight of them now feature serious GOP candidates who will raise real money and run real campaigns.

If Republicans can win three of those eight seats -- and not lose their own in Georgia and Kentucky -- they win the majority in November. That's entirely doable -- and just became more so with Gardner's entrance into the Colorado race.