His state's caucus Saturday is just the start of the tough balancing act Reid faces this year as leader of a Senate whose work will be shaped by the race for the White House.
While neither Clinton nor Obama is expected to use the Senate as campaign central for their presidential bids, dealing with their outsized personalities and increasingly competitive agendas promises to complicate Reid's job of seeking a Democratic consensus on jump-starting the economy and the war in Iraq.
"The presidential contest and presidential ambitions now are just so overwhelming, making it extremely difficult for Harry Reid to manage all of it, particularly in the context of the narrow margins," said Charles O. Jones, a Brookings Institution analyst.
"On top of all of this, he's a majority leader essentially without a majority."
Reid's predicament was on vibrant display on his home turf, where a union loyal to Clinton filed a lawsuit against the state Democratic party to challenge caucus rules that allow voters to caucus at nine casinos on the Las Vegas Strip. Obama suggested the suit - which was thrown out by a federal judge on Thursday - was aimed at depressing turnout among the workers whose influential union has endorsed him. The case created a dilemma for Reid, whose silence was interpreted by some in Obama's camp as favoring Clinton.
Reid's aides acknowledge that staying above the presidential fray has sometimes been a challenge for the senator, especially in Nevada, where he plans to declare himself "uncommitted" when voters caucus on Saturday.
"He's staying out of it - he's steering clear," said Jon Summers, a Reid aide. "Anything that he does then can be read as a tilt toward either campaign. ... There's a fine line you have to walk there to make sure that one (candidate) doesn't think you're talking to the other too much."
Still, there has been much speculation about where Reid's loyalties lie. His oldest son, Clark County Commission Chairman Rory Reid, is a top aide to Clinton's campaign. Reid also has a long history with Bill Clinton, who dispatched Cabinet members to campaign for him during a tough 1998 re-election race. That has made it difficult for Reid to tamp down the perception that he is quietly favoring the New York senator.
"He's in the middle on this one. He brought the caucus here, but now he has these brushfires he has to deal with," said David Damore, a University of Nevada, Las Vegas political scientist. "If the race goes on for a while and gets more intense between the two sides, it makes it more difficult for him to try to conduct the Senate's business."
Reid's challenge is likely to persist after his state's caucuses are over, strategists and congressional aides said, as Clinton and Obama compete for endorsements from their colleagues and for the spotlight on major policy debates.
"The job skill that he brings to this is the same one that he used in prior tight Senate races where you had a large number of senators running for re-election. His job is to make sure that whatever gets before the Senate is what has the broader support in the (Senate Democratic) caucus, and it's obviously harder with two of his colleagues in direct competition," said Jimmy Ryan, a former Reid aide.
Reid had months of practice in balancing senators' presidential aspirations, before Sens. Joe Biden of Delaware and Chris Dodd of Connecticut dropped out of the race.
Now he has little choice but to stay on the sidelines in a contest that is already dividing Senate Democrats, strategists said. His two top deputies have publicly taken opposing positions, with the No. 2 whip Sen. Dick Durbin of Illinois working to boost Obama, while Sen. Charles E. Schumer of New York, who heads the party's Senate campaign committee, sides with Clinton. Sen. Patrick J. Leahy of Vermont Thursday became the latest senior Democrat to enter the fray with his endorsement of Obama.
Reid "wants things to work in the Senate, and his driving, operating force is, what can he do to make sure that he gets more seats in the Senate and that his senators are happy," said Jenny Backus, a Democratic consultant and former Reid aide. "He's known the Clintons for a long time - they've been good to Nevada - but I think he's also smart enough to realize what electoral power Barack can bring."
Some Democrats noted that Reid has ties to both campaigns, including sharing a top political adviser, Jim Margolis, with Obama.
"His people are spread across the campaigns, as they should be," said Democratic consultant Steve McMahon.
"When you have two people in the caucus who are running against one another and they have various colleagues who are supporting them, as leader you want to do everything you can to stay neutral.
It's important for the caucus," McMahon said.
Reid's job will get harder the longer the Democratic contest goes, strategists said. Many expect a nominee to emerge quickly - perhaps as early as the first week of February, when 24 states pick a nominee. If the competition continues, though, Reid will have to deal with increasingly stark fissures among Democrats.
Once a nominee is chosen, Reid - who will be tasked with coordinating the 100-member Senate's work with the candidate's message - still faces a daunting challenge in trying to muster the votes to address any major issue.
The Senate's 49 Republicans also will be tailoring their agenda to their nominee. The gridlock that Reid weathered in 2007 from Republicans using filibusters and threats of them to block and stall Democrats' agenda will likely become even more evident in coming months.