After a rollicking four months of legislative acrobatics, Senate Majority Leader Harry Reid (D-Nev.) helped cap off President Barack Obama’s first 100 days with a bang. But the challenges for Reid in making sure Obama’s ambitious agenda is realized don’t end just because Pennsylvania Sen. Arlen Specter decided — in part, at Reid’s urging — to switch to the Democratic Party.

It’s a move that brings Reid’s Conference one step closer to the 60-vote majority that it needs to kill GOP filibusters, considering Specter’s defection Tuesday gives the Democrats 59 votes. Minnesota Democrat Al Franken would make 60 if he prevails in the legal battle over his race with former Sen. Norm Coleman (R).

But Reid indicated that 60 votes is not a panacea.

“I certainly don’t count [Specter] as an automatic vote,— Reid said Tuesday. “Not surprisingly, I don’t count anyone in my caucus as an automatic vote.—

Indeed, Reid still has perhaps the trickiest job on Capitol Hill, particularly because he will have to work even harder to keep a band of widely divergent Democrats together to pass Obama’s big-ticket items, like health care reform and climate change legislation.

What’s more, with a filibuster-proof majority, Republicans are likely to charge that Democrats will have no one to blame but themselves if they can’t get Obama’s agenda across the finish line.

“They might have a 60-Member majority. That doesn’t mean they have 60 votes,— said Sen. Ben Nelson (D-Neb.), who has been a key swing vote in the past. “It will be issue by issue that will determine how many votes the majority party gets.—

[IMGCAP(1)]Specter has already said he cannot be counted on to vote for a union-organizing bill known as “card check— and indicated Tuesday that he may vote against the Democrats’ budget resolution because it protects any health care bill from a filibuster.

Then there’s Nelson, who already has proved that he’s willing to follow his independent streak. He caused a stir earlier this year when he joined with three Republicans and withheld his vote on the $787 billion economic stimulus bill until Democratic leaders agreed to cut more than $100 billion from the measure’s price tag.

Of course, Nelson and Specter won’t be the only ones whom Reid will have to pacify. Any Democratic Senator could trip up legislation if he so chooses, and until Franken is seated, as is expected, Reid will still have to persuade at least one Republican to support controversial legislation. The Minnesota Supreme Court won’t take up the latest legal challenge until June.

However, Reid is well-practiced in overcoming those hurdles, even if he’s struggled at times and had to make tough concessions for Republicans and Democrats, such as on the stimulus bill.

“I’ve been here 25 years, and there’s no question these first three months are the most productive by far I’ve ever been involved in,— Reid said in an interview Monday. “We’ve produced a significant amount of legislation with little or no help from the Republicans.—

Indeed, the Senate is having an easier time in the 111th Congress. Stronger Democratic numbers, along with a popular Democrat in the White House, have made it easier to overcome the 60-vote hurdle on many Obama priorities, including a children’s health insurance bill, an equal pay measure, the omnibus spending bill that funds the government and a national service measure. The Senate is poised to pass a budget blueprint today, but that measure is privileged and only needs a simple majority to pass.

Plus, aides point out, Reid has made other strategic decisions designed to optimize his ability to get the votes that he needs to shut down GOP filibusters. One of those decisions was in ignoring liberal calls to oust Sen. Joe Lieberman (ID-Conn.) from the Democratic Conference after he campaigned for then-GOP presidential nominee Sen. John McCain (Ariz.) and was highly critical of Obama and the Democrats.

Just as he drew former Sen. Jim Jeffords (I-Vt.) from the Republican Party into the Democratic fold in 2001, Reid has been courting the moderate Specter for five years.

Though other Democrats were involved in persuading Specter to defect, Reid’s soft sell over the years “laid a path for Specter to be able to do this quickly and easily,— one senior Senate Democratic aide said.

Reid is loath to take the credit for the Senate’s rapid pace and success this year, saying the White House has been a strong partner in helping to secure Democratic and Republican votes.

“Obama has been a help not only with his strong appeal to the American people — his popularity — but he has been a help in the trenches,— Reid said.

And the self-professed pessimist sounded optimistic this week that his efforts to reach out to Republicans, which so far have largely been rebuffed, would end up benefiting Democrats in the end.

“The efforts of what I have done, what Obama has done, have given us very little short-term success, but long term, I think it’s going to pay off,— he said.

And after Specter announced his switch, Reid told reporters that he did not plan to change his tack with the GOP. He has already told Minority Leader Mitch McConnell (R-Ky.) that he will not move to shut down the GOP’s ability to offer amendments on the floor. Even without the ability to block bills by filibuster, Republican objections could still significantly slow bills on the Senate floor.

Reid’s successes in Washington, D.C., still do not appear to be translating back home, where he is up for re-election in 2010 and has seen his popularity wane.

Reid’s defenders, however, said his strengths are not necessarily as a communicator.

“Where he’s the most effective is as a dealmaker. He’s not going to be the best communicator, but if you want deals to be made, things to get done, trains to run on the track, you call Harry Reid,— the senior Senate Democratic aide said.

Though he clearly isn’t the most eloquent speaker, Reid can also count among his recent successes the avoidance of some of the verbal gaffes — such as calling former President George W. Bush a “loser— and a “liar— — that have caused problems for him in the past. But, Reid said, that hasn’t been by design.

“I’ll continue to be descriptive when I feel it’s necessary,— Reid said. “I’m not trying to behave myself.—