Republicans have a message for Mitt Romney: It’s time to go positive.

Prominent party leaders, unsettled by the frequently combative tone of Romney’s presidential campaign, are pressing the presumptive Republican nominee to leaven his harsh criticism of President Barack Obama with an optimistic conservative vision that can inspire the party faithful, appeal to swing voters and set out a governing agenda should he win in November.

Their worry: that the angry tenor of the Republican primary season could carry over into the general election, leaving Romney trapped in a punch-counterpunch campaign that would limit his ability to define fundamental differences with the Democrats. In interviews, these Republicans said that Romney must focus more on what he is for, not just what he is against.

“Mitt Romney has to come up with a plan and policy and principles that people can rally around,” said Gov. Gary R. Herbert of Utah, a strong supporter of Romney who said it was “fair game” to point out differences with the president. “It can’t just be negativity.”

The assessment of the candidate’s style, which the campaign has so far resisted, carries special weight because it comes from many of Romney’s best-known supporters, like Jeb Bush, the former governor of Florida, and Mitch Daniels, the governor of Indiana.

In interviews, Republican leaders said they agree with Romney’s attacks and understand that he is trying to harness the anger of the Republican base. But they said he has not yet struck the right balance between explaining what is wrong with his opponent’s record and what is admirable about his own.

The goal, they said, should be to capture the sunny conservatism embodied by Ronald Reagan and to a lesser extent George W. Bush, neutralizing liberal efforts to portray Romney as biting and backward-looking.

The issue is important for Romney because he has often had difficulty talking about his conservative principles without sounding forced and off key (describing himself as “severely conservative”) or creating policy problems for himself (calling for illegal immigrants to self-deport). And it does not help that polls show Obama starts with an advantage among voters when it comes to likability.

When Rick Snyder, the Republican governor of Michigan, endorsed Romney recently, he delivered this advice: “The American people want to hear a positive vision for America.”

“You want to have a situation where people have a reason to vote for you, not a campaign based on why not to vote for somebody else,” Snyder said.

William Kristol, the conservative writer, gently chided Romney this week in the pages of his magazine, The Weekly Standard, for engaging in small-bore squabbles with the president, including a withering speech in Charlotte, N.C., that the Romney campaign billed as a “prebuttal” to Obama’s address at the Democratic National Convention.

“Instead of giving rebuttals and prebuttals to Obama’s speeches, Romney can give serious speeches about the Constitution and the Supreme Court, the case for limited government and the threat of bankruptcy and penury, about undoing Obamacare and what will replace it,” Kristol wrote.

“Romney should run for president rather than run against Obama,” he wrote.

Romney has touched on some of those issues on the campaign trail, but they are rarely the thrust of his speeches.

The sustained nastiness of a presidential nominating contest traditionally gives way, eventually, to a finer-tuned, more upbeat message in the general election. But Romney was forced to wage a longer and harder nominating fight than expected, which could make it tricky for him to quickly pivot into a gentler mode.

And in the days since Rick Santorum dropped out of the race, effectively crowning Romney the nominee, the former Massachusetts governor has only intensified his attacks on Obama, traveling to North Carolina, Pennsylvania and Ohio to deliver often slashing assessments of what he has called the president’s “record of failure,” his habit of “punishing people” and his plan to “attack success.”

In an interview, Daniels said, “I do think he ought to shift gears now.”

He added, “Very few Americans need anyone, Governor Romney or anyone else, to tell them much about the president’s record. It’s there for them to see. I don’t think there is any particular tactical reason for Governor Romney to keep belaboring that.”

Several Republican leaders said they are not hearing enough of Romney’s own plans for fixing what’s broken.

Jeb Bush said that Romney “needs to stay above the fray.”

In an interview a few days ago with Newsmax, a conservative media organization, he called on Romney to “offer a hopeful message that can lift people’s spirits, because after the end of this four or five months of really negative campaigning, I think people are going to be motivated by a more positive message.”

It is a sentiment echoed by several voters attending Romney’s campaign events. Linda Heck showed up at a factory in Lorain, Ohio, late last week to hear Romney explain his plans for the economy. She was disappointed that much of his speech ticked off a list of grievances against the president.

“I didn’t hear a lot of ‘This is what I’m going to do and this is how I’m going to do it,”’ Heck said.

Romney’s aides privately acknowledged that they must find a way to evoke the kind of optimism that can attract moderate Republicans and independent voters. (Romney seemed to take a half-step in his speech Tuesday night, after winning five primaries, declaring that “Americans have always been eternal optimists.”)

“Mitt Romney is going to present a positive plan for how he would govern and contrast that with President Obama’s failures,” said Eric Fehrnstrom, a senior campaign adviser to Romney.

The campaign shows no immediate sign of shifting from attack mode, precisely the approach some supporters want him to maintain. Terry Branstad, the Republican governor of Iowa, said Obama was the candidate running the negative campaign.

“He is spending his time personally attacking Romney,” said Branstad, who encouraged Romney to keep highlighting the president’s shortcomings.

“I believe if we do that, we will win this general election.”

Herbert, the Utah governor, said that he wanted to hear Romney discuss a topic he routinely skirts, for fear of reminding voters of his prodigious wealth: his successful career.

Romney, he said, should frame his financial success as a totem of the America he is fighting to restore — a free-market economy, unburdened by overregulation and big government, in which entrepreneurs thrive and, in turn, employment grows.

“He has been way too timid about talking about his successes in the private sector,” Herbert said. “It’s what’s great about America. I can be the next Bill Gates or Mitt Romney.”

© 2012 The New York Times Company