CAPITOL HILL: This is a story of ifs. If the GOP wins the Senate. If the GOP wins they still have to woo six Democrats to get important legislation passed. If the Obama administration decides to play hardball after the election.

It’s a lot of ifs. But as of now, the New York Times electoral map gives the Republicans roughly a 65 percent chance of taking the Senate. That would mean a Republican would take the chairmanship of the Senate Armed Services Committee. And our understanding is that McCain would not be subject to GOP term limits that would otherwise block him from leading the SASC. And he is the senior GOP senator on the committee.

And Sen. Lindsey Graham, McCain’s bosom pal on many issues, has a strong position on both the Senate Appropriations defense subcommittee and the appropriations committee as a whole. Graham has a good shot at becoming chair of the appropriations State and Foreign Operations subcommittee. That would give him a powerful lever over Obama administration foreign policy and some national security issues.

If the GOP takes the Senate, then McCain is virtually a shoo-in for the SASC chairmanship. The current ranking member, Sen. James Inhofe, will probably take the chairmanship of the Environment and Public Works Committee, leaving the way open for McCain, who is the GOP’s most senior member on the SASC.

“In practice the Senate is pretty much straight seniority,” a well-heeled defense lobbyist noted.

What would McCain’s ascendancy mean for the armed services committee, and for the Pentagon? While he’s best known to the public for his sometimes feisty public comments, McCain and outgoing SASC Chairman Sen. Carl Levin share a great deal of the same views on defense issues, notwithstanding the usual partisan silliness they all have to engage in to raise money and prove they are loyal to the tribe.

McCain is especially known for his commitment to oversight, the congressional job of keeping the rest of the government honest. That won’t change. And his inclination to push for improved acquisition and to save taxpayers money would dovetail nicely with the efforts of the likely chair of the House Armed Services Committee, Rep. Mac Thornberry. Thornberry is pushing hard to build a new blueprint for Pentagon acquisition, working with Frank Kendall, the head of Pentagon acquisition. McCain could well join with his fellow reformer, even across the great gulf that divides the House and the Senate.

Would Thornberry and McCain be able to lead a rush against sequestration and convince the leaders of their chambers to lift the looming burden, I asked the lobbyist? He was not hopeful, even with all the talk there’s been about defense lawmakers pressing the House and Senate leaderships for a compromise to belay the mandatory budget cuts.

“Even if the GOP has a great night with 54 seats, you can get some stuff done but you still need six Democrats to get anything important done — and you’ve pretty much just destroyed the ones most likely to work with you,” the lobbyist says ruefully.

Then there’s the Obama Administration. Should the Republicans win the Senate they won’t be able to act unilaterally, no matter how self-satisfied and happy they are with their victory: “That is the $120,000 question: what posture does the administration take the day of the first session?”

Mackenzie Eaglen, with the American Enterprise Institute, outlines some of the limits McCain will face even if the GOP does take the Senate. “McCain’s agenda depends in large part on two men and what they want to achieve: Obama and [the likely Senate Majority Leader Mitch] McConnell,” Eaglen says in an email. “If Obama wants to get something done, he can and he will. And he may just find a willing partner in Republicans (eg, trade promotion authority).”

And McConnell will face his own limits and choices. “McConnell must decide whether his tenure is one where he shows the GOP can govern and work with the President or whether the GOP works to differentiate themselves from Obama leading up to 2016 and more gridlock ensues,” Eaglen adds.

Even if McCain becomes chair, he will face other obstacles, beyond those of the administration and the Democrats. He faces reelection in 2016.