WASHINGTON (Reuters) - For months, Republican Mitt Romney's presidential campaign has been built on broad themes: cut taxes, repeal and replace Democratic President Barack Obama's healthcare overhaul, increase defense spending.

But when it comes to specifics - namely, how to pay for the tax cuts and extra spending, and what exactly a Romney healthcare plan would look like - Romney has been reluctant to give details, essentially gambling that Americans' frustration with high unemployment rates and a struggling economy will be enough to propel him to the White House.

Now, with polls showing that Obama has taken a slight lead in the race after the Republican and Democratic national conventions, increasingly anxious conservatives are calling on Romney to spell out more of his plans - even if it risks alienating some undecided voters.

The calls for a change in strategy have become particularly loud since Sunday, when Romney struggled during an interview on NBC's "Meet the Press" program to explain what income tax loopholes he might close to help offset the cost of his tax cuts, or whether he would keep portions of Obama's healthcare overhaul, including a requirement of insurance coverage for those with pre-existing medical conditions.

"Mr. Romney's pre-existing political calculation seems to be that he can win the election without having to explain the economic moment or even his own policies," said an editorial published Tuesday in the Wall Street Journal, which often is a barometer of the thinking of leading conservatives.

"Such vagueness carries its own political risks," the Journal editorial said.

It isn't the first time that conservatives in his party have raised doubts about Romney's campaign strategy, but with the November 6 election less than two months away, the calls for the former Massachusetts governor to be bolder and more explicit have become increasingly urgent.

William Kristol, editor of the conservative Weekly Standard, said that Romney could be setting a course to lose the election despite the factors working in the Republican's favor - such as the nation's 8.1 percent unemployment rate.

"When a challenger merely appeals to disappointment with the incumbent and tries to reassure voters he's not too bad an alternative," Kristol wrote, "that isn't generally a formula for victory."

Romney's advisers say they are sticking with their strategy and not panicking. A Romney adviser from outside the campaign said there was nothing to be gained by putting out a specific plan on issues such as his tax-cut proposal because it would have to be negotiated with Congress.

"When he's president, it might call for him to put out a more specific plan to negotiate with them. But there's no reason for him to put out a detailed tax reform plan now," the adviser said. "It'll just allow the Obama campaign to shoot at it and not put out a plan themselves."

REPUBLICAN HAND-WRINGING

Romney has long had trouble winning over many of the Republican Party's most ardent conservatives, a problem that was evident during a long and bruising primary campaign.

He is distrusted by some conservatives largely because of moderate stances he took as governor of liberal Massachusetts from 2003 to 2007, when he backed a state healthcare overhaul that was a model for Obama's nationwide plan.

Obama's post-convention "bounce" - which put the Democrat ahead in what had been an even race - may be short-lived. But it has ignited a wave of Republican hand-wringing about Romney's campaign team and his failure to flesh out his conservative positions more boldly.

Romney's campaign is "too intent on winning over the small batch of uncommitted and independent voters by saying absolutely nothing that might possibly offend them," John Podhoretz, a conservative columnist and former presidential speech writer for Ronald Reagan, wrote in the New York Post.

"The problem with that strategy is a) it means he doesn't say much, and b) it does nothing to stimulate the enthusiasm of those already in his corner," Podhoretz said.

'SHUT DOWN THE PARTY'

Conservative radio host Laura Ingraham said Monday that if Romney's campaign fails to capitalize politically on the nation's sluggish economy, the implications for the Republican Party should be lasting.

"If you can't beat Barack Obama with this record, then shut down the party," she said. "Shut it down. Start new, with new people. This is a 'gimme' election, or at least it should be."

As dire as such analyses make it seem for Romney, the presidential race remains close. An online Reuters/Ipsos poll on Tuesday showed Obama with a 3 percentage point edge on Romney, 46 percent to 43 percent. The two candidates were tied on who would do the best job handling the economy.

The criticism Romney is facing from within his party is similar to the concerns some Democrats expressed about Obama's campaign in early September 2008, after Republican John McCain charged out of his convention on a wave of momentum and, with vice presidential nominee Sarah Palin, seized a slight lead in opinion polls.

At the time, Obama tried to reassure supporters and retooled his message to take a more aggressive approach. Shortly afterward, the worst financial crisis since the 1930s hit the United States, boosting Obama's calls for change after the eight-year tenure of Republican President George W. Bush. The Democrat cruised to a relatively easy 7-point victory over McCain.

Romney, a former private equity executive whose vast holdings in offshore accounts has led Democrats to accuse him of dodging taxes and call for him to release more than the two years of tax returns he has made public, has echoed that argument in refusing to release more returns.

He has said that doing so would merely give Obama's allies more targets for criticism. Romney's stance has, however, added to criticism of his campaign's tactical decisions.