"It's going to make a big difference if Mitt Romney is the V.P." for McCain, said Nick Isenberg, a Democratic activist from Glenwood Springs. "We have a lot of Mormons in Colorado."

Romney is a member of the Mormon church, officially known as the Church of Jesus Christ of Latter-day Saints.

Colorado is going to be an uphill struggle for Obama, Isenberg said, even if his campaign ranks it with Virginia at the top of the list of states the Illinois senator hopes to flip.

"We've been voting Republican for a long time," Isenberg said, and a lot of Coloradans - especially independents - still don't know much about Obama.

"It's a tossup here, and there are so many undecideds," Isenberg said. "We need to get people registered, and we're working like crazy."

U.S. Sen. Ken Salazar, D-Colo., conceded in an interview that Colorado is "a tough state for Democrats, and it's always been a tough state."

Romney performed well in many Western states during the GOP primaries, but his faith proved to be a big liability among the evangelical Christians who make up the base of the Republican Party. Many are deeply suspicious of Mormons, forcing Romney to give a speech explaining his beliefs in an effort to quell their concerns.

For the general election, the West, especially the Southwest, rises in strategic significance for both candidates, and Mormons are gaining more attention given their wide dispersion across the region. Although church members are heavily concentrated in Utah, where they make up more than 70 percent of the population, according to church figures, they also top 7 percent of Nevada's population and 2 percent of Colorado's, enough to tilt a tight race.

The Southwest is pivotal to Obama's hopes to change the electoral map. His strategy rests on breaking the decadeslong, but weakening, Republican grip on Colorado, New Mexico and Nevada. That becomes especially important if he fails to wrest from the GOP any of the Southern states, led by Virginia, he has been targeting.

Democrats have started a political action committee, the Western Majority Project, to push for a "Democratic realignment" in the West by building on the party's extraordinary gains in the fast-growing region over the past six years.

Still, McCain is narrowly ahead in three of the four most recent polls in Colorado, all conducted in mid-August, before Obama chose Delaware Sen. Joseph Biden as his running mate. The candidates are splitting recent polls in New Mexico, and McCain is ahead in two of the three latest polls in Nevada.

"Obviously, if he picks Romney, it makes a strong play for the Mormon vote, but I don't know that that decides the West," said Arizona Gov. Janet Napolitano, an immensely popular Democrat in McCain's home state. In an interview, Napolitano said Romney's reputation for changing his position on issues will not play well with Western voters, who she said tend to like independent pragmatists.

What matters more, Napolitano said, are Latinos, who constitute more than 38 percent of the voters in New Mexico and about 27 percent in Arizona. She argued that McCain's disavowal of his own failed attempt at immigration reform will cut into his support with this more powerful voting bloc.

She called McCain's own popularity in Arizona "anemic." Although McCain is expected to carry his home state easily, Napolitano noted a surprisingly large number of undecided voters.

Salazar predicted McCain will be hurt by his comment this month that he wanted to renegotiate the multistate compact dating to 1922 that distributes water from the Colorado River, presumably to allocate more water to Arizona.

"It would be something you might have said in front of a Phoenix Rotary Club," he said. "We absolutely ... have been doing everything we can to protect our compact for us to keep our water here in Colorado."

Political operatives said such regional issues, often overlooked by national campaigns, can be critical in presidential races.

"You're not really running a national campaign - that's another fallacy," said Bill Hillsman, a media consultant who worked with former Minnesota Gov. Jesse Ventura and Green Party candidate Ralph Nader, among others. "You're running statewide and regional campaigns. You can't look at it as a national race; that's not what decides who wins or loses, as we found out in 2000."

E-mail Carolyn Lochhead at clochhead@sfchronicle.com.

This article appeared on page A - 7 of the San Francisco Chronicle