By Steve Holland

BOULDER, Colorado (Reuters) -- Seasoned politicians trailing Donald Trump and Ben Carson for the Republican presidential nomination are eager to shift the campaign focus to the economy and policy in Wednesday’s debate and expose what they see as weaknesses in the two front-runners.

Jeb Bush and other candidates are trying to turn the tide in a campaign that is dominated so far by provocative rhetoric that has played to the strengths of Trump, a bombastic reality television star and developer, and Carson, a soft-spoken surgeon who has been gaining support in opinion polls.

The two-hour debate, moderated in Boulder, Colorado, by business network CNBC, will begin at 8 p.m. EDT Wednesday.

With Trump and Carson holding a firm grip on the race in polls of likely Republican voters for the November 2016 election, the forum comes at an increasingly perilous time for lower-ranking candidates.

Former Florida Governor Bush, former Hewlett-Packard CEO Carly Fiorina, Ohio Governor John Kasich, former Arkansas Governor Mike Huckabee and New Jersey Governor Chris Christie are under pressure to shake up a race for the party's nomination that so far is tilting away from them with the first voting to take place in little more than three months.

Texas Senator Ted Cruz and Florida Senator Marco Rubio also need solid performances to build on recent momentum.

Officials from several rival campaigns said they believe the debate could help make Trump and Carson less popular if they are shown to lack knowledge of the intricacies of policy.

“If they run this thing well and push people to see if they're smart on the economy and job creation and how fiscal restraint fits into that, you could finally start separating the sheep from the goats on an important issue," said an official in the campaign of one of the Republicans vying against Trump.

Republican strategist Kevin Madden said the debate could pose a test for Trump and Carson.

"It requires them to no longer just glide by on attributes like being new and bold," said Madden, a former top aide to 2012 Republican nominee Mitt Romney.

Carson set the table for a debate about the future of sacrosanct entitlement programs by telling "Fox News Sunday" he would use health savings accounts as an alternative to popular Medicare and Medicaid health programs for the poor and elderly.

Trump, suddenly behind Carson in some polls, went on the attack on MSNBC's "Morning Joe", saying he did not think Carson would get away with "abolishing Medicare", which Carson denied he would do.

In Westerville, Ohio, on Monday, Kasich signaled he would take a tougher tone with Trump and Carson.

“I want you to know I’m fed up. I’m sick and tired of listening to this nonsense and I’m going to have to call it like it is in this race,” he said.

A Trump aide said the billionaire would be well-prepared to respond to attacks at the debate. A Carson spokesman did not immediately respond to a request for comment. 

Without mentioning Trump or Carson specifically, the campaign of Huckabee, who has offered a detailed "fair tax" plan, said the debate will require candidates to go beyond talking points.

"All of them as a whole are going to have to give more than just the topline bullet points of their economic policies," said Huckabee spokeswoman Alice Stewart.

Trump frequently touts the business experience that made him a billionaire as reason enough why a President Trump would create a stronger economy.

He released a tax reform plan in September that would lower tax rates for all Americans and would pay for the loss of tax revenue by eliminating tax deductions and corporate loopholes.

The non-partisan Tax Foundation said it would reduce tax revenues by $10.14 trillion over the next decade when accounting for economic growth from increases in the supply of labor and capital.

Carson has proposed all Americans pay a flat tax of 10 percent on income based on the biblical notion of tithing. He would eliminate individual and corporate tax loopholes. He has said his proposal would be revenue neutral for the federal budget.

Bush has been active in taking on Trump, and an aide said he plans to do so in Boulder.

Bush may have little choice.

"Jeb is trailing in the polls, he's got nothing to lose," said David Yepsen, director of the Paul Simon Public Policy Institute at Southern Illinois University.

For more on the 2016 presidential race, see the Reuters blog, “Tales from the Trail” (here)

(Additional reporting by Erin McPike; Editing by Caren Bohan and Ken Wills)