This week’s Senate Commerce, Science and Transportation hearings on global climate change, prenatal genetic-testing technology and pornography addiction may seem, at first glance, to be just a way to fill time during the lame-duck session.

But for Sens. John McCain (R-Ariz.) and Sam Brownback (R-Kan.), the hearings are likely the last chance both men will have to delve into those issues as a chairman on Commerce or one of its subcommittees.

“When you give up a chairmanship, you lose the ability to set the agenda,” noted McCain in an interview last week.

In the new Congress, McCain is set to be replaced as Commerce chairman by Ted Stevens (R-Alaska), who currently chairs the Senate Appropriations Committee. As the incoming chairman of the Senate Indian Affairs Committee, McCain will no longer be able to call in scientists to discuss how pollution and other factors are warming the globe.

Still, McCain said a recent report showing a rapid thinning of the Arctic ice shelf and its potentially detrimental affect on polar bears and seals made him want to hold this Tuesday’s hearing — likely the last of his tenure atop Commerce.

Of course, McCain’s seniority on the full committee will still allow him to remain a player on Commerce as chairman of the communications subcommittee — a post he has said he’ll take over from Sen. Conrad Burns (R-Mont.) in the new Congress.

Because of that high-level switch, Brownback — a much less senior member of the panel — will likely lose his primary outlet for delving into controversial social issues, such as stem-cell research, genetic testing, pornography and abortion.

Indeed, McCain’s decision to take over the communications panel will likely spur a downward shuffle of subcommittee chairmanships that is likely to knock Brownback out of the top spot on the science, technology and space subcommittee.

Under the rubric of “technology,” Brownback — an ardent opponent of abortion rights — has stretched the jurisdiction of the committee to suit his interests in stem-cell research and prenatal genetic testing, which have been linked to the abortion debate, Senate aides in both parties say. Brownback has scheduled hearings on genetic testing and pornography addiction for Wednesday and Thursday, respectively.

Indeed, Sen. Frank Lautenberg (D-N.J.), a subcommittee member, has often attended Brownback’s hearings for the sole purpose of chiding him for what Lautenberg sees as a misuse of the subcommittee’s jurisdiction.

“Senate committees should not be manipulated to launch a personal crusade,” said Lautenberg in a statement.

In the next Congress, Brownback is expected to have a harder time holding those types of hearings as chairman of either the competition, foreign commerce and infrastructure subcommittee or the consumer affairs and product safety subpanel, which will be his two remaining choices given his seniority on the panel.

However, one Senate aide pointed out that the consumer affairs panel could prove useful in exploring some divisive social issues as they relate to pornography and possibly even abortion.

Brownback spokesman Brian Hart said it was “way too early to tell” which panel Brownback might choose, given that none of the subcommittee chairmanship decisions has been officially made.

In addition, McCain won’t necessarily be shying away from controversy at the helm of communications. Incoming Chairman Stevens has stated his desire to rework the 1996 Telecommunications Act during the 109th Congress.

Because McCain and Stevens have long had an adversarial relationship — primarily over McCain’s opposition to Stevens’ alleged “pork barrel” spending — it remains to be seen whether the two can work together on such a massive rewrite dealing with emerging telecommunications technologies.

McCain has even mused about the possibility that Stevens could try to block him from taking the communications post, noting: “The last word resides with the chairman.”

Lending some credence to this possibility is the fact that committee rules say subcommittees may be added, removed or revamped during a change in the full committee chairmanship — and that “seniority on the particular subcommittee shall not necessarily apply.”

Still, McCain said he believed Stevens would adhere to the custom of allowing senior members of the panel to choose their subcommittee chairmanships.

“I think it’ll work fine. He’s a very strong individual and so am I,” said McCain. “You don’t have to be best friends around this place to get things done.”

Stevens’ spokeswoman, Melanie Alvord, said the chairman could not be reached for comment.

McCain’s seizure of the communications subpanel from Burns puts Burns in line to chair the aviation subcommittee, which is considered the second most attractive subcommittee in Commerce. Several calls to Burns’ office last week were not returned, however.

If Burns takes aviation as expected, then Sen. Trent Lott (R-Miss.), the current aviation chairman, would be forced to choose another top spot. Insiders say that would likely be the surface transportation and merchant marine subcommittee chaired by Sen. Kay Bailey Hutchison (R-Texas).

Lott already used his seniority on Commerce to commandeer the aviation panel away from Hutchison at the beginning of the 108th Congress. That came in the wake of Lott’s forced resignation as Senate Majority Leader. Now, aides say Lott is interested in the surface transportation gig because it includes oversight of shipbuilding, a key industry in Mississippi.

A Hutchison aide said the Texas Senator would be just as happy chairing Brownback’s science, technology and space subpanel, given that her state is home to NASA’s Johnson Space Center in Houston as well as influential defense and space contractors. Still, she holds out hope for keeping surface transportation or regaining the chairmanship of aviation.

Next in seniority is Sen. Olympia Snowe (R-Maine), who chairs the oceans, fisheries and Coast Guard subcommittee. Snowe’s office said they know of no one interested in taking the panel away from her in the 109th.

Depending on what Brownback chooses, Sen. Gordon Smith (R-Ore.) could then lose his chairmanship of the competition, foreign commerce and infrastructure subcommittee.

But Smith can thank Sen. Peter Fitzgerald (R-Ill.), who’s retiring at the end of this Congress, for the fact that no current Republican subcommittee chair will lose a subcommittee chairmanship in the 109th. Fitzgerald chaired the consumer affairs and product safety subcommittee, which would be available to Smith if he loses his current slot.