When Montana's governor, Brian Schweitzer, launched his campaign for re-election this summer he chose to be pictured on a horse lassooing a calf, and in silhouette against a barn door, presumably after a long hard day on the ranch. What was not mentioned was the word Democrat - a party affiliation that in some parts of the west still conjures up associations with gay marriage, gun control and abortion.

Those sensibilities could be in for a shake-up as Barack Obama launches the Democrats' most aggressive drive for years for votes from the Rocky Mountain west.

The strategy unrolling now across the west could turn once reliably Republican states such as Colorado, Nevada, New Mexico, and possibly even Montana into America's new electoral battleground. But it is Colorado which is viewed as the biggest prize.

"Colorado defines competitive," said Dick Wadhams, the chairman of the state Republican party. "I think either party could carry Colorado."

It all hinges on how you define Democrat - a political label that has been comprehensively altered by a new generation of elected politicians in the west - and an impressive grassroots organisation now being assembled by the Obama campaign.

The big push begins next week, when some 50,000 Democrats are expected to converge on Denver for the party's convention. Democrats see the convention, especially the spectacle surrounding Obama's nomination speech at a giant football stadium, as a chance to impress on voters in Colorado and other states the party's renewed commitment to the west.

Obama's first appearances on his return from holiday this week were in Nevada and New Mexico, and he is planning to campaign in Colorado after the convention. "Colorado and the west are both a big focus of this campaign," said Stephanie Mueller, a spokeswoman for the Obama campaign in Denver.

Not that John McCain, the Republican candidate, is expected to yield easily. McCain has lived in Arizona for the last 25 years, and has made frequent visits to Colorado and Nevada.

But after eight years of an unpopular George Bush, Democrats believe that voters in Colorado and other states are disillusioned with Republicans. They are frustrated with the rising deficit, the war in Iraq, and worried about the economy. This could be Obama's moment.

Unlike previous elections, when the Republicans rode a wave of emotion over terrorism, or issues such as gay marriage and abortion, this time voters' concerns are down to earth: the economy and the need for renewable energy sources.

That has led to a blurring of the conventional political divisions. In Colorado, unaffiliated voters outnumber Democrats and Republicans. Soaring petrol prices have turned conservative farmers into ardent conservationists. Even the Republican candidates have taken to carrying around solar film panels and using public transport to get to campaign events.

The early signs are encouraging for Obama. The two candidates are virtually tied in Colorado, although a Rasmussen poll last week put McCain ahead of Obama in the state for the first time in seven months. McCain and Obama are also running neck and neck in Nevada and New Mexico.

But Obama claims the edge in organising, grafting his campaign on top of rejuvenated local organisations.

In every state of the west, the Obama campaign has taken the lead in voter registration, increasing the number of Democrats.

In New Mexico, the campaign has opened 20 offices - compared to just five in the 2004 elections - and the organising effort has extended into rural areas that have not been involved in presidential contests for decades.

In Colorado, Obama's campaign has opened 10 offices, including in towns seen as redoubts of the Christian right.

Even in Montana, a state of 950,000, the campaign has deployed 40 paid organisers to register new voters.

Wadhams insists that much of the excitement about Obama's organising prowess is over-stated. "There is a lot of smoke and mirrors," he said.

But Democrats across the west believe enthusiasm about Obama's candidacy and the campaign's investment in the region could make this the Democrats' moment.

"That investment and the energy that we see on the ground in New Mexico will absolutely translate in November," said Brian Colón, the chairman of the New Mexico Democratic party.

The challenge for Obama though will be to resist Republican efforts to brand him as an elitist, or a textbook liberal more at home in San Francisco or New York than in the west.

For the last 40 years, the west has sent only Republicans to the White House - except for 1992 when Bill Clinton benefited from a third party candidate who drained Republican support. By 2000, Republicans governed all eight states, and controlled 13 of the 16 Senate seats.

Then Democratic activists began to plot their comeback. In Colorado, local entrepreneurs began funnelling money towards liberal causes, building up institutions that could put Democrats on a surer footing. First came the thinktanks, then media monitoring organisations. By 2004, the local renewal efforts under way in Colorado and other states had a powerful patron in Howard Dean, the Democratic national committee chairman.

Under Dean, the Democrats began putting money and resources into building up party networks even in states which had little prospect of voting a Democrat into the White House. Western Democrats credit the strategy with gaining a foothold for the party even in staunchly Republican states such as Utah and Wyoming, and say it will prove its value over the coming years.

"In a place like Montana there simply weren't the resources to keep a party headquarters opened during the entire year in non-election years and so it was very difficult," said Schweitzer. "It kept people on the ground recruiting new candidates, maintaining voter files, making sure we have county committees that are working and organised."

At the same time, the Democrats began recruiting a new generation of organisations, and began scouting for political candidates who could appeal to moderate voters in the west.

Eight years on, five of the eight governors of western states are now Democrats, and five of the 16 senators.

Betsy Markey, who is hoping to unseat a Christian right member of Congress in Colorado, has a simple formula for success that Obama might want to heed. What makes a successful Democrat in the west? "The government not telling you how to live your life and what to do," she said.