The public implosion of Energy and Commerce ranking member Joe Barton (R-Texas) has made it almost certain that the Republican side of the committee will have a new leader next year — and there is no shortage of volunteers gunning for it.

Nearly all of the most senior GOP members of the committee have expressed interest in the post, but two, Reps. Fred Upton (Mich.) and John Shimkus (Ill.), have emerged as early favorites, according to Members and outside observers.

Even before his infamous apology to BP, Barton was in danger of losing the position because his term as the top Republican expires at the end of the 111th Congress. He had hoped to get a waiver from the House Republican Steering Committee that would allow him to extend his tenure, but that appears almost unthinkable now.

Upton and Shimkus are popular within the conference and have donated time and money to help the party regain the majority through the National Republican Congressional Committee.

Upton, the third-ranked Republican on the committee, is known as a measured, thoughtful lawmaker who is willing to compromise with the other side of the aisle.

On the fundraising front, Upton co-hosted the NRCC fundraising dinner in March and has been “involved at every level” of the campaign committee, according to one House Republican Member, who requested anonymity.

Asked about his interest in the post, Upton said his first focus was wining seats back in 2010, but he also said he has made his personal goals clear.

“I said for sometime that my focus is trying to take back the House,” Upton said. “I’m not on the Steering Committee, so I don’t have a vote on whether [Barton] gets a waiver to stay or not … but I think folks know I’m interested in being a post-Barton guy.”

While less senior on the panel than Upton, Shimkus is seen as a strong, conservative contender for the position in part because of his leadership on energy issues.

Shimkus, who hails from coal country, has emerged as a key Republican voice on the committee on issues such as cap-and-trade, while also maintaining a solid working relationship across the aisle with fellow coal district Rep. Rick Boucher (D-Va.) and other Democrats.

“It’s clear, by every indication, that this’ll be Rep. Barton’s last term as the top Republican on the Energy and Commerce panel,” said one former GOP leadership aide with close ties to the energy industry. “Rep. Shimkus seems to be the best, and leading, candidate to assume the ranking member post — or with hope, the chairmanship — in the next Congress.”

Steven Tomaszewski, a spokesman for Shimkus, said in an e-mail that the lawmaker would pursue the position if it becomes available.

“Congressman Shimkus’ first priority is making sure John Boehner is the next Speaker,” he said. “If an opening were to occur, Congressman Shimkus would pursue a ranking/chairmanship.”

But Upton and Shimkus face different sets of obstacles that they will need to overcome in order to ascend to the top Republican position on the panel.

In Upton’s case, his moderate voting record has caused some conservatives to question whether he would be a strong leader for the committee.

While his middle-of-the-road views reflect those of the people in the southwestern Michigan district that he represents — 54 percent of them voted for President Barack Obama in 2008 — some lawmakers say it could disqualify him from the leadership slot.

“Upton has a credential problem with conservative members,” said a second GOP lawmaker who spoke on condition of anonymity. “He’s not always a reliable vote on hard issues, where Shimkus is all of those things.”

The lawmaker noted that Upton’s views on abortion in particular are a problem for some Republicans.

Upton refuted the charge and listed his votes on several anti-abortion issues, including those he cast in favor of Rep. Bart Stupak’s (D-Mich.) amendment to the health care bill that blocked any federal funding for abortions.

Upton has also occasionally sided with Democrats on energy issues.

For example, in May 2008, Upton was one of 35 Republicans to vote for the Democratic bill that extended tax credits to support research into alternative energy.

“Upton has been working hard now, but he’s got challenges to get over,” the second GOP lawmaker said.

Although Upton ranks behind Rep. Ralph Hall (Texas) on the panel, few observers on or off the Hill believed seniority would be one of Upton’s obstacles.

Even so, Hall told Roll Call in an interview that he planned to seek the post.

“I want to be the chairman if we take the [House],” Hall said. “If we don’t, I’ll stay ranking, I hope.”

For Shimkus, the hike to the top committee spot is a steep one from a seniority perspective.

“Seniority does play a role,” the first House Republican Member said. “Someone who is junior has to justify why they are jumping over four or five others, [and] usually there has to be a fairly substantial reason for that to occur.”

In order to claim the post, the Illinois Republican would have to leapfrog over Hall and Upton, as well as Reps. Cliff Stearns (Fla.) and Ed Whitfield (Ky.).

Spokesmen for Stearns and Whitfield confirmed both senior lawmakers would be interested in the top position on the energy panel.

At times, Shimkus has provoked public relations controversies of his own.

In a 2002 debate over the Yucca Mountain nuclear waste storage project, he brought a poster of a Nevada license plate featuring a mushroom cloud, depicting an atomic bomb test, onto the House floor.

“The state of Nevada can again fulfill its nuclear legacy and continue to aid this nation and our citizens” by accepting the storage site, said Shimkus, a supporter of nuclear energy. Some Nevada lawmakers, noting deaths that occurred as a result of the testing, said Shimkus’ comments were in poor taste.

In 2009, he walked out of the House chamber as the president addressed Congress on health care reform. He later apologized.