At the height of campaign season, and while under extra scrutiny as a potential vice presidential pick, Sen. Rob Portman (R-Ohio) is on a quixotic bid to reform how the miscellaneous tariff bill is crafted.

In the process, he’s going against the Republican Ways and Means Committee chairman — a close friend, as it happens — while giving bipartisan cover to a vulnerable Democratic Senator.

“I’m just trying to get something done here that helps the country,” Portman said in an interview in his office in the Russell Senate Office Building.

The miscellaneous tariff bill, or MTB as it is known inside the Beltway, is a collection of scores of narrowly targeted bills that temporarily suspend taxes on importing chemicals and other goods U.S. manufacturers hope to use for less cost.

Although relatively obscure, it’s a top priority of the business lobby. But the bill is in limbo because many critics of Congressional earmarks say the limited tariff benefits that make up the law are banned under House and Senate rules.

Portman, a veteran of both Bush administrations and the House, has joined Sens. Claire McCaskill (D-Mo.) and Jim DeMint (R-S.C.) in introducing legislation that would give the International Trade Commission the first look at the proposals, rather than vetting what Congress sends to the ITC, transcending the problem with the earmark ban.

Their bill has the backing of many Republican leaders. Senate Minority Whip Jon Kyl offered the bill as an amendment to Citrus Trust Fund legislation in a Finance Committee markup Wednesday.

“If we don’t modify the process, we’re not going to have any MTB this year,” the Arizona Republican said, adding that most Senate Republicans believe the tariff suspensions qualify as earmarks.

But Senate Finance Chairman Max Baucus (D-Mont.), whom McCaskill has described as fiercely protective of his committee’s turf, panned the bill, which lost on a voice vote.

“This process as I understand it — although it’s not been vetted — reduces transparency,” Baucus said, warning ominously of “lobbyists” influencing the ITC. “It’s a secret process.”

House Ways and Means Chairman Dave Camp (R-Mich.), Baucus noted, is “strongly opposed” as well, adding, “He thinks it’s a bad idea.”

Baucus and Camp had been pushing to move the MTB through without altering the earmark ban.

And as Camp was fending off pressure from conservative groups that the MTB plan amounted to pushing through earmarks, the Portman-McCaskill proposal was announced, undercutting his argument that no such change to the process was necessary.

The move caused tension between the two lawmakers, even though their friendship dates back to Portman’s tenure in the House, when they both sat on the Ways and Means Committee together.

“Dave Camp is not just a friend, he’s one of my best friends in Congress,” Portman said.

“He’s legitimately concerned that if we try to put these reforms in place this year that we’re gonna miss a year on the process of approving these tariff bills. And I agree with his concern. We tried to answer that in the legislation by expediting the process for this year. And that’s where we’ve asked the ITC to expedite their review to get them back to Congress for a vote.

“I’ve been trying to respond to that very legitimate concern but at the same time make it clear that in my judgment — and maybe I’m wrong — but in my judgment, we’re gonna end up with no progress this year unless we have a reformed system.”

A spokeswoman for Camp pointed to the Finance Committee vote as evidence “that we have strong bipartisan support for the transparent MTB process under way in both the House and Senate.”

Meanwhile, Portman’s partnership with McCaskill has raised eyebrows because the Missouri Democrat is facing real peril in her re-election bid.

Facing an uphill battle in a state that President Barack Obama is not expected to contest seriously, McCaskill has cultivated a staunchly independent profile, including breaking from her party on key issues that hold populist appeal.

Sen. Roy Blunt (R), her home-state colleague, participated in working groups on the bill, but he never signed on as a co-sponsor.

“I’m not particularly supportive of the way that language turned out,” Blunt said in a brief interview, adding that the political implications for McCaskill’s campaign had nothing whatsoever to do with his decision not to sign on.

Portman, too, said politics had nothing to do with the issue.

“Look, this is a political year, no question about it,” he said. “I’m not absent from the campaign trail, as you know, but this is about getting something done that helps American workers.

“I think we shouldn’t worry about the politics. They’re gonna sort themselves out. This is not gonna make a big difference in any election. This is not gonna change the dynamics of any race, national or state. This is a relatively small matter in the context of the big concerns we have as a country. But it’s an important one,” Portman said.

Meanwhile, the intense speculation on Portman as a frontrunning contender to be presumptive GOP nominee Mitt Romney’s vice presidential pick has focused attention on his record and personality.

Portman said he did not know whether Romney had decided on a vice presidential candidate yet and declined to discuss the campaign’s vetting of him, aside from recounting some of the more clever methods reporters have used to query him about it.

He also declined to perform an impression of Obama his aides said was dead-on. “Not a chance,” Portman said.