Colorado holds primaries Tuesday to select the top two contenders to succeed Gov. John Hickenlooper, a centrist Democrat whose promotion of aerospace, tech and a plethora of other industries helped generate unprecedented economic growth in this rapidly-growing state of 5.6 million people. But to win, the leading Republican and Democratic candidates are eschewing the middle ground in this heavily independent purple state to appeal to their respective bases. Republican Walker Stapleton has wedded himself to President Donald Trump, and Democratic U.S. Rep. Jared Polis is tacking left on universal health care and marijuana. That may be a risk: This year, independents — voters not affiliated with any party — can vote in the Republican or Democratic primaries. While independents are the largest voting bloc in Colorado, analysts say it's too soon to measure their impact on primary outcomes. Still, to hear Stapleton tell it, he's already past the primary and running against Polis. Stapleton, a two-term state treasurer, lashed out at Polis, a five-term Boulder congressman, several times during a Republican gubernatorial debate Tuesday — on immigration, raising taxes for schools and roads and safety standards for oil and gas drilling in a rapidly expanding Denver metropolitan area. 'I've taken a de minimis amount of money from people in the energy industry, but guess what — I hope they're listening, because it's going to need to be a lot more to defeat Jared Polis,' Stapleton said, referring to Polis' $12 million investment in his own campaign and advocacy of local control over Colorado's $31 billion oil and gas industry. On that issue, Polis, a tech entrepreneur and one of the wealthiest members of Congress, advocates strict safety standards and full-throttle investment in green and renewable energy. He is a longtime advocate of Colorado's burgeoning marijuana industry and eliminating federal interference. But he's also fighting a surprisingly close primary race centered on public education issues against former state Treasurer Cary Kennedy, leaving it to the state Democratic Party to tackle Stapleton. In all, four Republicans and four Democrats want to succeed the term-limited Hickenlooper. It's the top primary race in a state that hasn't elected a Republican governor since Bill Owens, who served from 1999-2007, or opted for a Republican presidential candidate since George W. Bush in 2000 and 2004. Republicans hold a 4-3 advantage in Colorado's congressional delegation and one of two U.S. Senate seats. They control the state Senate and serve as treasurer, attorney general and secretary of state. In the primary run-up, Republicans and Democrats offered starkly different post-Hickenlooper visions for Colorado's role — or resistance — in implementing Trump administration policies on immigration, the environment, taxes and health care. In recent days, Hickenlooper himself has barred state agencies, including the National Guard, from supporting immigrant family separations, and he ordered Colorado to adopt California's strict vehicle pollution rules. Stapleton bear-hugged the administration's deportation policies just as immigrant family separations were causing a national outcry. So, too, did his GOP rivals Victor Mitchell and Greg Lopez. Only Doug Robinson, a nephew of former presidential nominee Mitt Romney, opposed them at the Denver Post-KMGH-TV debate. 'It's not who we are as Americans,' Robinson said. 'We are the party of family values.' It's an uphill climb for Stapleton, a favorite of the GOP establishment whose mother is a cousin of President George H. W. Bush. Mitchell, who's invested nearly $5 million in his campaign, challenges Stapleton's truthfulness, especially his claim — since abandoned — to be the only U.S. state treasurer to endorse Trump's income tax cuts last fall. The Democratic race has focused on protecting immigrant rights, strengthening the Affordable Care Act and pressing state concerns such as underfunded schools and roads and skyrocketing housing costs. All vow to amend constitutional tax-and-spending restrictions that hamper investment in schools and transportation — a goal that proved elusive for Hickenlooper. Kennedy has run a strong grass-roots campaign embraced by Colorado's largest teachers union. A former Denver deputy mayor, she authored a constitutional amendment designed to raise K-12 spending. Both Polis and former state Sen. Mike Johnston also have extensive education credentials. Former New York City mayor and gun-control advocate Michael Bloomberg has invested in Johnston's campaign, which has featured roundtables on gun violence. Also running is Democratic Lt. Gov. Donna Lynne, a former high-ranking health care executive who avoids the campaign squabbling, preferring a measured and sophisticated insistence that Colorado's challenges have no easy answers. 'I think the election is for sale,' Lynne said this week as her opponents bickered over their campaign finances. 'I'm a workhorse, and not a show horse.