Republicans are expressing fresh concerns that Mitt Romney is limping toward the presidential nomination, suffering new blows at the very moment he needs to grow stronger if he is to take on President Obama in November.

Even before Rick Santorum’s surprising sweep of three contests on Tuesday, the Romney campaign was receiving a steady stream of advice — and warnings — from Republicans who are increasingly anxious about Romney’s performance, which has not improved over nearly six weeks since the state caucuses and primaries began.

One prominent adviser told the candidate to sharpen his use of conservative code words and create “small pictures” — vivid imagery, in other words — to connect with voters. Another flew to Boston to say that Romney’s message is too businesslike and broad to capture the passion of angry Republican voters. Still others have gone on television and written opinion columns to hammer home what is becoming a common theme this year: that Romney has not been able to ignite a cause when the GOP is primed to become part of one.

After Romney’s three wins, five losses and his solid lead in delegates, most Republicans said they still think he will win the nomination.

But they see a candidate who lacks broad support among conservatives, and whose recent defeats reveal that his organization is not quite as unstoppable as many supporters had thought.

What appeared to be a sure win may become one of Romney’s toughest tests. The Feb. 28 primary in Michigan — his home state and the place where his father served as governor — is shaping up to be a much more difficult race for him. Despite his extensive network and widespread name recognition, Romney must contend with a new surge of popularity for Santorum, a former senator whose modest background in western Pennsylvania may resonate with Michigan’s embattled autoworkers. In a strike from the other direction, Democrats in Michigan are criticizing Romney over his opposition to the federal bailout of the auto industry.

“It’s a bit of a wake-up call for Governor Romney,” said one of the candidate’s advisers, who spoke on the condition of anonymity to be candid. “He’s obviously got to sharpen the message and clarify the vision — particularly for activists who are more likely to participate in these contests.”

After Tuesday, Romney’s team appears to be listening. There is agreement, one adviser said, that the candidate must liven up his road show, injecting energy into a stale stump and resuming the kind of town-hall events that let him interact with voters. He will hold his first such event since Jan. 13 on Friday in Portland, Maine. The state’s caucuses will end Saturday.

Romney is not expected to move too far to the right to accommodate the party base, the adviser said, adding, “You can’t make him who he’s not.” All of the Romney advisers interviewed declined to speak for attribution, saying they are not authorized to do so.

The changes under discussion appear relatively modest and may not alter the dynamics of the race. Romney intends to continue his heavy focus on the economy — an area that is considered Obama’s greatest vulnerability — and to try to demonstrate his command of facts and policy on issues such as Social Security, Israel, housing and the environment, one adviser said. His supporters have called for a deeper evaluation of his message, saying he lacks the kind of inspirational rallying cry that could begin to shut down his rivals.

Even Romney’s supporters acknowledge that he cannot afford to lose another contest right away. He is pushing hard in the Maine caucuses, followed by primaries in Arizona and Michigan on Feb. 28.

On Thursday, Romney appeared to temper hopes about Michigan.

“I’m not expecting a landslide. I can’t tell you 100 percent that I’ll win,” he said in a Fox Business Network interview. “I’m planning on it, and I’m going to work hard for it. And I think the people of Michigan understand that this is the chance to select as our nominee a person who was born and raised in Michigan, who understands Michigan values and who will do everything in his power to get Michigan working again.”

Romney noted that, in 2008, Sen. John McCain (Ariz.) lost 19 states before winning the GOP nomination.

A strong view remains among many Republicans that Tuesday’s nonbinding contests in Colorado, Minnesota and Missouri, in which no delegates were awarded, were less damaging to Romney than news reports have let on. The real loser Tuesday, many of these Republicans said, is former House speaker Newt Gingrich, who was banking on a two-man race with Romney and now must compete with Santorum for the conservative mantle.

“They are playing Santorum and Gingrich off each other,” said one outside adviser, referring to the Romney officials. “They’re basically like whack-a-mole. This week, they’re whacking Santorum; last week, it was Gingrich. The goal is to keep the anti-Romney vote 100 percent split.”

There is also a strong view that Romney should be careful not to stray too far from who he is.

“The worst thing you can do is to be something you’re not,” said Rep. Thaddeus McCotter (Mich.), who ran for president before endorsing Romney. “I think Governor Romney knows that. He’ll continue to be Governor Romney. The last thing Americans need to see in the Republican primary is for people to try to pretend to do something they’re not.”

Still, the need for that whack-a-mole strategy, several Republicans said, speaks to Romney’s seeming inability to excite the GOP’s conservative base.

As if to acknowledge the point, the candidate met privately with leaders at a conservative political conference in Washington on Thursday before his address there Friday.

Republicans also fear that Romney’s scorched-earth strategy may drive down the voter enthusiasm he would need for a fall campaign.

And that’s why the advice has rolled in, some of it contradictory: to hit Santorum harder; to stop hitting Santorum; to talk up his management experience; to stop running as a manager and start using the language of a movement; to rehire Brett O’Donnell, a debate coach who was fired last week after Romney advisers bristled at the credit he received for the candidate’s debate performances in Florida; even to stop saying “enterprise” when he means “business.” (“Does anyone know what that means?” one adviser asked. “The public thinks ‘enterprise’ is either a car-rental place or Captain Kirk’s ship. It’s just a lot of annual-report talk as opposed to kitchen-table talk.”)

“He’s been trying to win this with overwhelming throw weight — with more money, negative ads and manpower,” said a former Romney adviser, Alex Castellanos. “They’re trying to win this tactically in each state, separate from the other, which gets expensive and long. Either he elevates his purpose, he elevates from a campaign to a cause, or the big Romney bulldozer has to now turn, grindingly slow and powerful, and crush everything in its path.”

Staff writer Philip Rucker contributed to this report.