DENVER -- Republican strength in this year's House and Senate races could, strangely enough, hurt the party's presidential chances by stalling the changes in style and policy advocated after Mitt Romney's defeat in the 2012 presidential campaign.

GOP officials and strategists say it's hard to persuade party leaders to adjust the political recipe when they feel increasingly upbeat about adding Senate control to their solid House majority this fall. This optimism, numerous GOP strategists say, makes looking past the party's loss of the popular vote in five of the last six presidential elections easy.

"It's very difficult to make an argument for change and modernization when you're winning," said Joel Sawyer, a former South Carolina GOP official who advises campaigns in several states. Citing the party's nationwide reliance on older white voters, Sawyer said, the GOP needs "to start modernizing now to become relevant to younger voters and nonwhite voters."

The party's dilemma was in sharp relief in a Denver public television studio here, where four candidates gathered for a Republican primary debate in the race to represent the deeply conservative, rural and exurban 4th Congressional District, which covers the eastern third of the state.

All the candidates said they oppose gay marriage, want to repeal President Barack Obama's health care plan and object to allowing people living in the country illegally to become citizens.

The front-runner in the Colorado race, Weld County District Attorney Ken Buck, narrowly lost a U.S. Senate race in 2010 because he was seen as too extreme on issues like abortion and immigration. Now he has been hit in ads by state Sen. Scott Renfroe for "flip-flopping" on those two issues.

"It's one of the great ironies of Republican politics that we fall victim to," Buck said in an interview. "I'm very conservative on life, I'm very conservative on immigration, but given enough money" anyone can be attacked for not being pure enough.

Renfroe said: "We need candidates who will stand firmly for what they believe."

Immigration was an issue singled out by a GOP-commissioned "autopsy report" last year that analyzed Romney's loss to Obama. The report said Republicans must embrace "comprehensive immigration reform"-- Washington shorthand for legalizing the immigration status of those living here illegally--to improve the GOP's strained relationship with the fast-growing Hispanic and Asian electorate.

The recommendations quickly hit resistance from congressional Republicans who rely on primary voters strongly opposed to "amnesty" for immigrants living here illegally.

The resistance might have softened if Republican lawmakers felt threatened by public disdain for the 2013 government shutdown or by worries of midterm election setbacks this fall. Instead, the shutdown issue faded, and Republicans are having a good primary season.

With all cylinders firing, "it's harder to hold that conversation about making the party a winner at the national level," said former Senate Republican staffer John Ullyot.