Early Drive for Clinton Unsettles Democrats
Some Worry It Will Siphon Money from Candidates in Midterm Elections

"Clinton's biggest vulnerability in 2008 was the aura of inevitability," said Ben LaBolt, who worked in both of President Barack Obama's presidential campaigns. "That could be compounded in 2016—particularly if there's no serious primary opponent—and so a slow and deliberate ramp up could serve her well."

The formidable campaign apparatus that has sprung up to support a possible 2016 presidential bid by Hillary Clinton is rattling some Democrats, sparking concerns that it could suppress competition for the party nomination and siphon money from candidates running in the midterm elections this fall. With Democrats fighting to keep control of the Senate in the midterms, the emergence of the pro-Clinton super PAC Priorities USA could eat into donations that the party's candidates need to win in November, some Democrats warn. A number of Democratic groups are soliciting donors for money, including super PACS devoted to House and Senate races along with traditional party fundraising committees.

Aware of the anxiety, Priorities officials are considering tailoring their requests so that donors would be asked to write their biggest checks after the midterm elections are over, people close to the super PAC said. For example, Priorities might ask a donor to pledge $1 million to the Clinton effort over the next few years, but write a check for only $100,000 this year, a person familiar with the discussion said.

"My goal is to not have donors honestly say, 'I can't send money to you, House and Senate [candidates], because I'm giving all this money to Priorities,' " a person close to the process said.

Vice President Joe Biden and Hillary Clinton are mulling presidential runs. Associated Press

More broadly, if Mrs. Clinton glides to the nomination without a serious challenge, some Democrats say, the party would be deprived of an important debate on issues that have split the party, including government surveillance, trade pacts and how best to keep Iran from developing a nuclear weapon.

"We need a vibrant, competitive primary process where not only can people sharpen their positions on different issues, but also get ready for what's going to come in the fall [2016 general election], which is going to be brutal," said Dick Harpootlian, a former Democratic chairman in South Carolina, which traditionally holds one of the first presidential primaries.

Mr. Harpootlian said he visited Joe Biden, the other most prominent potential 2016 contestant, in the White House about a month ago and told the vice president he would support him if he decides to run. Mr. Biden told him he hadn't yet made up his mind, Mr. Harpootlian said.

Some Democrats maintain it isn't even good for Mrs. Clinton herself to be seen as inevitable so early. She seemed similarly unstoppable early in the 2008 presidential race, only to find that voters cooled on her—a case, some analysts thought, of "Clinton fatigue."

"Clinton's biggest vulnerability in 2008 was the aura of inevitability," said Ben LaBolt, who worked in both of President Barack Obama's presidential campaigns. "That could be compounded in 2016—particularly if there's no serious primary opponent—and so a slow and deliberate ramp up could serve her well."

Mrs. Clinton has said she hasn't made up her mind about running for president and won't do so until later this year. A Clinton spokesman on Sunday declined to comment on concerns about her front-runner status.

As she considers her options, several outside groups have arisen to help smooth the way. Foremost among them is Priorities; Jim Messina, who managed the Obama-Biden campaign in 2012, has signed on as co-chairman of the group. A few other top Obama campaign aides also have aligned themselves with Mrs. Clinton, putting Mr. Obama on the spot as he tries to remain neutral.

A group called Correct the Record is rebutting Republican attacks directed against her and other possible Democratic presidential candidates. The project is an offshoot of American Bridge 21st Century, a liberal super PAC founded by David Brock, a one-time conservative-turned liberal watchdog.

Ready for Hillary is a super PAC that is collecting email address and identifying grass roots supporters in an effort to draw Mrs. Clinton into the race. A senior adviser is Craig Smith, who served in Bill Clinton's White House. The group held a pair of fundraising events in Philadelphia last week. Tickets for one of the events, aimed at young professionals, started at $20.16, with people offered the title of "co-host" or "vice chair" for bringing in more money, the invitation shows. More than 350 attended, a spokesman for the group said, while a separate cocktail reception netted in the "six-figure range."

No other prospective Democratic candidate has an infrastructure approaching this scale, and polling shows Mrs. Clinton has an enormous lead over possible rivals. Some believe the pro-Clinton machinery may serve to clear the field, discouraging others from challenging Mrs. Clinton.

David Gergen, who served in Bill Clinton's White House and also worked for Republican presidents, said: "There's no question that the Hillary movement is taking the oxygen out of the air for every other potential candidate." He mentioned Maryland Gov. Martin O'Malley as "the kind of candidate who in ordinary times might attract very positive press attention" but is getting little.

Chris Lehane, a Democratic strategist who worked in the Clinton administration, said early coalescing around Mrs. Clinton could benefit Democrats in 2016. "If you're looking at this from the perspective of what puts you in the best position to win or lose as a Democrat in 2016, it's clearly having the Republicans having to go through an incredibly contentious primary process whereas the Democrats have a very straightforward process," Mr. Lehane said.

The appearance of a Clinton juggernaut has stoked some resentment. Mr. Messina privately told the vice president's office of his plans to join Priorities before the news broke last month, according to people familiar with the matter. But some close to Mr. Biden felt hurt nonetheless that Mr. Messina would attach himself to a pro-Clinton group before the vice president made a decision on the race, one person familiar with the matter said.

Mr. Messina declined to comment.