WASHINGTON — Leading presidential candidates may be letting American taxpayers off the hook for financing their campaigns, but the public will be paying a steep price for the 2008 race in another way: Secret Service protection.

Federal officials expect to shell out more than ever before to keep the major party candidates out of harm’s way next year, due in large part to the wide-open field of hopefuls shaping up so far.

Estimates in the president’s budget proposal indicate that the government is allocating more than $100 million for protection in the 2008 election, more than twice as much as it projected for the campaign four years ago. That includes a $35.6 million increase requested in the budget released Monday.

The estimates add to what is fast becoming the most expensive presidential election in history. Senators Clinton and Edwards, top contenders for the Democratic nomination, have said they will forgo public financing of their campaigns in the primary and general elections, and more candidates in both parties are expected to follow suit so they can raise enough money to compete.

The Secret Service points to history as a reason for the record security funds: For the first time in the four decades since the agency began protecting presidential candidates, the election will lack an incumbent president or vice president who already carries a full detail of bodyguards.

Each party has several prominent contenders, and while none will receive Secret Service protection for months, if not a year, the agency is keeping an eye on the campaign. “We would be remiss if we didn’t pay attention to what’s going on. It starts very early,” a Secret Service spokesman, Eric Zahren, said yesterday. “We don’t pay attention to the politics of it, but we do pay attention to developments as far as the number of candidate details that we ultimately have in the picture.”

The Secret Service has been responsible for protecting major party candidates since 1968, when the law was changed immediately following the assassination of Senator Robert F. Kennedy. Campaigning against Vice President Hubert Humphrey and Senator Eugene McCarthy for the Democratic nomination, Kennedy was gunned down June 5, 1968, by Sirhan Sirhan hours after winning the key California primary.

While the secretary of homeland security, which oversees the Secret Service, is ultimately in charge of deciding which candidates qualify for official protection, Congress plays a small role. By law, the secretary must consult with an advisory committee that includes the House speaker, the House minority leader, and the majority and minority leaders of the Senate. The committee is supposed to consider fund-raising and public opinion polls in making the determination. The law stipulates that candidates and their spouses receive protection “not more than 120 days” before the general election, but in practice it has started much earlier. The Secret Service assigned agents to Senators Kerry and Edwards — the only qualifying Democratic candidates — in February 2004, nine months before the election.

The 2008 election cycle has begun even earlier, but a spokesman for the Department of Homeland Security, Jarrod Agen, said no determination had been made on when Secret Service protection would begin. The congressional advisory committee has not met, a spokesman for the House speaker’s office, Nadeam Elshami, said. Despite the lack of an incumbent running in the 2008 race, one candidate, Senator Clinton, already has a Secret Service detail, since she is the wife of a former president. Mr. Zahren said her protection would continue through her campaign, but he would not say whether she would receive additional bodyguards if she qualified under the normal candidate process. The Clinton campaign would not comment on the senator’s security arrangements or whether she expected greater protection as a candidate.

Presidential candidates without Secret Service protection usually have some kind of security presence, whether through their official positions or paid with campaign funds.

For the Secret Service, the presence of several particularly high-profile candidates could pose a unique security challenge. Mrs. Clinton is aiming to be the first woman president, and Senator Obama, a Democrat of Illinois, is trying to be the first African-American elected to the White House. The agency does not consider race or gender on its own in planning its protection measures. Mr. Zahren said officials look at a range of factors to form an overall security picture.