The re-elections of Sens. Blanche Lincoln of Arkansas and Arlen Specter of Pennsylvania won't hinge on the climate issue, but if a bill makes it to the Senate floor, these two moderate Democratic incumbents facing tough primary challenges could be walking an even finer line than they already are.

"It would become a front-burner issue," Pennsylvania GOP Senate candidate Pat Toomey told NationalJournal.com. "People would be much more engaged and more concerned about the bill if they saw that passage was imminent." Toomey is the Republican candidate likely to take on Specter or his Democratic challenger, Rep. Joe Sestak, in the general election.

Polling data shows Specter and Lincoln holding an edge over their primary challengers. Both primaries will take place on May 18. Senate Majority Leader Harry Reid, D-Nev., has continued to say he hopes to bring a climate bill to the floor this summer. So if Specter and Lincoln both make it past their primaries and if legislation makes it to the floor before November -- two big ifs -- the incumbents could find themselves boxed in politically ahead of the general election, given their voting records on previous climate bills.

As a member of the Environment and Public Works Committee, Specter voted in favor of the climate bill that the panel approved along party lines in November -- which contained more stringent and comprehensive carbon controls than are likely to be included in the package set to be introduced Monday by Sens. John Kerry, D-Mass., Lindsey Graham, R-S.C., and Joe Lieberman, I/D-Conn. If a climate bill comes to the floor, either Specter votes for the bill and Toomey attacks him for supporting a "massive job-killer," or Specter votes no and faces accusations of flip-flopping.

"I think it's an important issue in this race to the extent that it relates very closely to job growth. That's the number one issue in this race," Toomey said. He added that it's "entirely possible" Specter could vote no after voting in favor of the EPW bill, given the incumbent's record of "flip-flops," particularly his party-switch.

"I don't expect climate and energy policy to be a real central issue," said Dan Hirschhorn, editor and publisher of Pa2010.com. "The X factor, though, is if it becomes a heated debate in Washington. If there is a bill that comes to the floor, everything changes, and this will be a huge issue."

Lincoln could face a similar quandary. Many experts predict she will vote no if a climate and energy bill comes to the floor, given her hesitancy to engage in climate negotiations over the past couple of months and her support of efforts by Sen. Lisa Murkowski, R-Alaska, to block EPA from regulating greenhouse gas emissions. More recently, Lincoln has drawn attention by proposing stringent new financial regulatory measures -- a move which many view as an attempt to appease the Democratic base in the midst of an unexpectedly tough primary challenge from Lt. Gov. Bill Halter (D). But by the time a climate bill is likely to come to the floor, the primary would be over, presumably freeing Lincoln (if she survives) not to worry as much about shoring up her left flank.

In any case, Lincoln did vote in favor of the climate bill sponsored by Lieberman and Sen. Mark Warner, D-Va., in 2008, which made it to the floor but didn't pass, so she could be accused of flip-flopping if she votes no.

In her campaign's first TV ad in March, Lincoln listed several Democratic initiatives she opposed, including cap-and-trade legislation. But she hasn't released an ad touching on that issue again, and University of Arkansas political science professor Andrew Dowdle says that's no accident. "That drop-off in subsequent rounds of advertisement shows the fact that it didn't get a lot of traction in the state of Arkansas," Dowdle said. "The ones here are about jobs, and that's really indicative of what the big issues are."

Even if legislation makes it to the floor, some political analysts predict it will play only a limited role in the elections. "It will be used as a piece of a larger puzzle," said Frank Maisano, an energy specialist at Bracewell & Giuliani, a law firm that represents companies in the oil and gas industry. "It will be a third or fourth piece to add to drive home the point that this person is out of touch with the constituency."

How large a piece of the puzzle climate and energy policy will have could largely hinge on what type of bill the Senate trio unveils next week, and what type of bill, if any, ultimately comes up for a vote. "It's not at all clear if something will hit the Senate floor," said Tony Massaro of the League of Conservation Voters. "And a lot of it depends on what it is that hits the Senate floor, if something does. I don't think we know the answer to that yet."

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