With the Obama administration and Capitol Hill Democrats abandoning middle-of-the-road compromises to pursue a more ambitious agenda, Republican Senate moderates are discovering their leverage lies in party loyalty.

Senate moderates in both parties have long acted as swing votes on pivotal bills. But this year, GOP centrists want the Democratic majority and White House to work harder for their support, and without more compromise, appear content to stay close to a diminished GOP minority.

“Since I was first elected to Congress in 1978, I’ve sought to work with Members of both parties to find sensible, workable solutions to the critical issues,— Sen. Olympia Snowe (R-Maine) said. “Frankly, I just don’t know any other way. President Obama is the sixth different president since I’ve been in Congress, and whether it was Jimmy Carter and Bill Clinton or George Bush and Ronald Reagan, I’ve conducted myself in the same manner.—

Many in the GOP feared that President Barack Obama’s popularity combined with Senate Democrats’ 58-seat majority would give Democrats a blank check to pass legislation, at least early on. But so far this year, big-ticket items such as the stimulus bill and omnibus have passed with only a few Republican defections, and the GOP has remained unified against other bills like the budget and the looming “card check— labor bill.

“To think eight weeks ago that everyone in our Conference would vote against the president’s first budget, everybody would have been blown away,— a senior GOP aide said.

That dynamic, in turn, has given GOP moderates such as Maine Sens. Susan Collins and Snowe, Pennsylvania Sen. Arlen Specter and Ohio Sen. George Voinovich a greater degree of influence over the legislative process — something they may not have envisioned.

For instance, Snowe, Collins and Specter joined with moderate Democratic Sens. Evan Bayh (Ind.) and Ben Nelson (Neb.) in making significant changes to the administration’s original economic stimulus package, folding in a number of key items that Republicans had been pushing.

Likewise, during the recent budget fight, Republicans were able to add a number of amendments to curb some of the administration’s policy plans. And, despite pressure from the administration and the House, Senate Majority Leader Harry Reid (D-Nev.) has resisted efforts to include sweeping energy reform language in the budget blueprint, in part because moderates in both parties have been cool to the idea.

Republicans attributed part of the moderates’ success to the fact that Obama and Reid have been content to push legislation through the chamber with just a narrow majority of support. They said the Democrats seem disinclined to bring more Republicans into the fold by tempering their legislative plans.

One GOP aide likened Obama’s handling of the recent budget fight to how former President George W. Bush pursued Social Security reform.

Following the 2004 election, Bush announced his intention to pursue broad reforms to the entitlement program, and he embarked on what largely amounted to an extended political campaign swing to promote his plan.

But Bush all but ignored the political heavy lifting that was needed in Washington to advance his proposal. As a result, then-Minority Leader Reid and House Minority Leader Nancy Pelosi (D-Calif.) were able to keep virtually all of their Members in line for a prolonged and aggressive counteroffensive. Without buy-in from moderates on Capitol Hill, Bush’s plan never got off the ground and it was soundly beaten back.

Likewise, Obama has powered up his campaign operation to push his budget blueprint, using his national apparatus to press his case on the issue. And while Democrats enjoy large enough power in both Houses to force the spending plan through, Obama’s national push did little to persuade moderates, and indeed he eventually lost Democratic votes in the face of unified opposition from the GOP.

“This is what the Bush administration did on Social Security,— a Republican Senate leadership aide said, adding that, “It’s easier to campaign with a candidate than it is with an issue.—

By not steering a message and policy course closer to the middle, Republicans argued, Obama and Democrats have given GOP leaders the ability to operate more freely as an opposition party and present more stark alternatives when lobbying moderates to stand with them.

“It makes it easier to the extent that we are still relevant in the debate,— the leadership aide said.

Additionally, moderates have in at least one case found that aligning themselves with the administration can come with a cost. Specter’s support for the $787 billion stimulus bill has played a major role in his sinking poll numbers at home. And with his 2010 election around the corner, Specter last month essentially used his position as a swing vote to kill the pro-union, card check bill, which is opposed by conservatives.