The Massachusetts Democrat is making his pitch with an almost religious fervor, pushing a message that’s equal parts saving the planet, national security and the economy. Many of his colleagues have been impressed by Kerry’s expertise and his passion in trying to push through caps on carbon when others would prefer to move onto a more limited, energy-only bill.

Yet it’s that same zeal that is making some swing-vote Democrats cringe at the thought of negotiating with someone they fear is tone-deaf to the political realities of their respective states — particularly in a difficult midterm elections year.

Kerry’s style, said Sen. John Rockefeller(D-W.Va.), is akin to being “pursued by a suitor, just as boys pursue girls.”

Rockefeller, who last week called on the Senate to ignore carbon limits, declined to say whether Kerry is the Democrats’ best messenger on climate, saying it was “sort of too direct a question” to answer.

“Well, he’s certainly a constant,” Rockefeller said. “The question is whether that’s good.”

“He’s so obsessed,” said one wavering Democratic senator who has been pursued by Kerry. “Clearly, it’s all climate, all the time with him.”

Kerry is well aware that his tactics may have turned off some of his colleagues.

“No one pretends this has been an easy issue,” he told POLITICO. “It’s been contentious for the 20 years we’ve been trying to pass a bill. Have I worked aggressively? Absolutely. But I’ve also listened hard.”

But Thursday’s climate-focused Democratic Caucus meeting left some senators grumbling that Kerry talked too much and didn’t listen enough. Although Kerry opened his presentation with Sen. Joe Lieberman (I-Conn.) by apologizing for being so aggressive on the issue and explaining how important he thinks it is, he then went on to speak much longer than either New Mexico Sen. Jeff Bingaman or Washington Sen. Maria Cantwell, the two other presenters at the meeting.

Senate Majority Leader Harry Reid (D-Nev.) had to postpone the discussion for another meeting this week.

“It took too much time presenting,” complained Finance Committee Chairman Max Baucus of Montana. “Senators like to talk.”

A Baucus aide said his boss wasn’t singling out Kerry, but there’s no question that the Democrats’ 2004 presidential candidate has taken on an outsize role on climate change since he grabbed the legislative reins about a year ago.

According to his staff, he’s had 292 meetings and phone calls with senators, 72 meetings with Obama administration officials and 46 meetings with top international and environmental leaders, including U.N. Secretary General Ban Ki-moon and former Vice President Al Gore.

Kerry said that he’s following direction from Democratic leaders and other senators who wanted him to meet with industry representatives and other groups from across the country.

“I’ve asked my closest friends in the environmental community to swallow hard on some pieces that matter enormously to members of our caucus, and I think most senators appreciate that,” he added. “I believe it’s been worth the effort to fight. I feel this issue very intensely. I care about it more than anything I’ve ever worked on in my life. And I’ve always said to my colleagues, ‘I know this is tough, and I know it’s not everyone’s favorite issue, but I only push because I care so much and I desperately want to see us tackle it together as a caucus with one voice rather than giving up.’”

“I’ve never met anyone more committed to an issue than John,” said Sen. Lindsey Graham (R-S.C.), who worked with Kerry for nine months before abandoning the talks because of immigration politics.

Graham took note of Kerry’s decision to meet last month on natural gas policy with T. Boone Pickens, the Texas oilman who funded the Swift Boat Veterans for Truth ad campaign that helped derail Kerry’s 2004 bid for the White House.

“John’s willingness to sit down with Boone Pickens speaks volumes about him as a person and his willingness to find a solution to this issue,” Graham said.

Several Senate Democrats praised Kerry for attempting to address their key concerns and meeting with their constituents. Sen. Kent Conrad said Kerry has met on “multiple occasions” with coal, agriculture and rural electric cooperatives in Conrad’s home state of North Dakota.

“Imagine, I go to these meetings and I’m asking for [the Environmental Protection Agency] pre-emption; I’m asking for offshore oil drilling; I’m asking for revenue sharing. Those are in the bill,” Alaska Sen. Mark Begich said. “That’s a good sign.”

In attempting to win over pro-drilling Democrats, Kerry has turned off fierce foes of the oil industry, like New Jersey Sen. Robert Menendez.

“I won’t say he didn’t listen to me and didn’t at least try to meet me halfway,” Menendez said. “I just think what he offered in my particular case isn’t something I can fully accept.”

Graham suggested that Kerry’s approach has its limitations, particularly when it comes to working with industry. Several business sources said they were in meetings earlier this year in which Kerry lectured them on the science of climate change rather than dealing with the hard details of the legislation.

“One of the things I can tell you is, John Kerry could tell you about climate change in his sleep,” said Graham. “He’s very knowledgeable. But when you get a bunch of utility guys in a room, they want to know the bottom line.”

In the aftermath of the BP oil spill, some Senate Democrats are frustrated that Kerry is holding up action to respond to the disaster in the hopes of getting a comprehensive climate bill. He’s told leaders in private meetings that he still sees a path to 60 votes on his climate bill, but he’s offered little evidence to show that he can get there.

“Typical John Kerry,” one Senate Democrat complained.

At a June 10 meeting with committee leaders who have a slice of the issue, senior Democrats in the room were ready to bring a narrower energy bill to the floor and allow the Kerry-Lieberman plan to come up as an amendment, which would significantly limit the debate on climate change legislation and diminish its chances for passage. But Kerry, along with Lieberman and Sen. Barbara Boxer (D-Calif.), objected, vowing to try to find a way forward on their plan — and leaving the caucus no closer to consensus.

“He’s refused to take no for an answer,” said Jim Manley, a spokesman for Reid, who Manley said has been “impressed” by Kerry’s efforts.

But calls to drop any type of carbon cap are growing among moderate Democrats just weeks before Democratic leaders plan to bring legislation to the floor, and Kerry’s colleagues doubt he can get the votes.

“I’d support it,” said Sen. Dianne Feinstein (D-Calif.). “I don’t see 60 votes for it.”

Majority Whip Dick Durbin (D-Ill.) said he’s concerned that if he heeds Kerry’s request to start the floor debate without 60 votes, the whole package could hit a brick wall on procedure, especially if no Republicans come along.

Kerry has never been the most popular senator in the Democratic Caucus — and some say privately that’s a factor working against him, particularly as he’s teamed up with Lieberman, who has his own reputation as a thorn in the side of liberals.

“He’s not the centrist who normally does the kinds of deals like this,” said a former Senate Democratic aide. “It’s unclear why he’s emerged as a central player on climate other than his interest in the subject. That’s not typically the way this is handled.”

Asked if she’s been pursued by Kerry and Lieberman, Sen. Claire McCaskill (D-Mo.) said with a grin: “I’m trying to avoid them.”

But Kerry can’t take all the blame, either. Boxer didn’t win any popularity contests when she led the Democrats’ climate push the last time around. And several Democrats have openly questioned who’s in charge of the legislative process this time — Kerry, Reid or President Barack Obama himself?

“He’s not leading it; I think there are three or four or five or six people that have bills,” said Sen. Carl Levin (D-Mich.). “He’s probably the leader of the leaders.”

Reid and Obama also upset Kerry’s own apple cart when they put immigration legislation on the table, giving Graham an easy out of negotiations in April and stealing some of the momentum just days before the bill’s unveiling. Health care, economy and Wall Street regulatory reform repeatedly stole the spotlight for much of Obama’s first year and a half in the White House.

“There’s not going to be one person to blame; it’s all of our responsibility,” said Sen. Mary Landrieu (D-La.). “I think Sen. Kerry has done as good of a job as anyone.”