Republican Senate candidate U.S. Rep. Cory Gardner waves to volunteers as he arrives at GOP campaign headquarters, in Greenwood Village, Colo., Monday, Nov. 3, 2014. Gardner is vying against Democratic Sen. Mark Udall, seat that could be a key to both control of the U.S. Senate and questions about the GOP's national viability. (AP Photo/Brennan Linsley)
U.S. Sen. Mark Udall, D-Colo., center, hugs Rep. Luis Gutierrez, D-Ill., who rallied for Udall during a campaign stop focusing on Latino-American issues, at Metro state University, in Denver, Monday, Nov. 3, 2014. Udall, is in the fight of his political life as he tries to stave off the Republican challenger U.S. Rep. Cory Gardner in a race that is seen as key to both control of the U.S. Senate and questions about the GOP's national viability. (AP Photo/Brennan Linsley)
Jill Biden, left, the wife of Vice President Joe Biden, campaigns with U.S. Sen. Mark Udall, D-Colo., center, and U.S. Sen. Michael Bennet, D-Colo., in Boulder, Colo., Saturday, Nov. 1, 2014. Biden joined Udall to campaign, three days before votes will be counted as Udall defends his Senate seat from Republican challenger Rep. Cory Gardner. (AP Photo/Brennan Linsley)
DENVER — Befitting its status as a national political bellwether, Colorado is home to at least three nationally watched political races that will be decided Tuesday.
Democratic Sen. Mark Udall is in the fight of his political life as he tries to stave off Rep. Cory Gardner in a race that is key to both control of the U.S. Senate and questions about the GOP's national viability.
Gov. John Hickenlooper, also a Democrat, is struggling to keep his job, as is Republican Rep. Mike Coffman in a suburban Denver contest that is considered one of the toughest U.S. House races in the country.
Finally, Colorado will test the power of a new election law pushed through by the Democratic Legislature that mails a ballot to all voters and allows citizens to register to vote until Election Day.
Former President Bill Clinton, one of the many national figures to stump in the state in the past few weeks, said: "Everybody in Colorado can vote. It's the easiest place in the country. So it's just a question of who wants it badly enough."
That might be Republicans.
The party hasn't won a top-of-the-ticket race in Colorado since 2004, when the state supported President George W. Bush's re-election. Since then, it's elected three separate Democratic senators, two governors and, twice, President Barack Obama.
In the GOP-friendly 2010 election, the party captured down-ballot offices like attorney general and treasurer. But it lost the gubernatorial race to Hickenlooper and a senate race to Michael Bennet, a Democrat whose surprise win hinged on women's issues.
Udall is trying to replicate Bennet's effort in his race against Gardner, while his challenger is seeking to link the incumbent to Obama, who polls show is unpopular in the state.
Hickenlooper, meanwhile, has been trying to stay above the fray, in keeping with his nonpartisan demeanor. He has emphasized Colorado's shrinking unemployment rate, which has dropped to 4.7 percent.
Republican Bob Beauprez has attacked the governor for what he sees as state government overreach and being soft on crime, including sparing a convicted multiple-murderer from execution.
Democrats have sought to replicate their recent Colorado success nationally.
Should Udall or Hickenlooper lose, it would show the party can't always survive a tough environment with the Obama-age coalition of young and minority voters.
But should Republicans lose — especially Gardner, perceived as their best candidate in years — it will be a grim signal for the GOP's viability in the state and other fast-growing ones like it.
The state's congressional races, meanwhile, have drawn little notice — with one exception.
That is Republican Rep. Mike Coffman's seat in Aurora and neighboring towns, the center of a pitched battle between him and former Colorado House Speaker Andrew Romanoff, a Democrat.
It is considered one of the country's most competitive congressional races. Coffman narrowly won re-election in 2012 in a district that largely supported President Obama.
Also getting attention in Colorado is a measure that would require the labeling of certain genetically modified foods. That proposal and one in Oregon would apply to raw and packaged foods produced entirely or partially by genetic engineering.
As is often the case, Colorado's down-ballot races have been overshadowed by the fireworks in its top two contests.
— In the attorney general's race, Deputy Attorney General Cynthia Coffman, a Republican, faces former Adams County District Attorney Don Quick.
— In the secretary of state contest, Joe Neguse, a Democratic University of Colorado Regent, is competing against El Paso County Clerk Wayne Williams, a Republican.
— For treasurer, Republican incumbent Walker Stapleton is being challenged by former Democratic Rep. Betsy Markey.
In the state's other congressional races, Democratic Rep. Diana DeGette is expected to cruise to re-election against Republican Martin Walsh in her Denver district.
Democratic Rep. Jared Polis is in little danger from Republican challenger George Lenig in his Boulder-centered district, and Republican Rep. Scott Tipton has good odds of beating Democratic challenger Abel J. Tapia in his western Colorado district.
Democrat Ed Perlmutter is expected to win re-election in Jefferson County over Republican Don Ytterberg.
In Colorado Springs, Republican Rep. Doug Lamborn was criticized for remarks interpreted by some as suggesting that military generals should resign to protest Obama administration policies. But his Democratic challenger, retired Air Force Maj Gen Irv Halter, faces an uphill battle in the overwhelmingly Republican district.
Likewise, Weld County District Attorney Ken Buck, a Republican, is the favorite to win Gardner's newly vacant eastern Colorado congressional seat. He is running against Democrat Vic Meyers.
Voters also will decide whether to allow expanded gambling in Aurora and whether to make the death of a fetus in a criminal act a homicide.