Both John McCain and Barack Obama have touted their ethical standards on the campaign trail, emphasizing that they are politicians who can't be bought.
And they appear to be conscientious of the fine line between constituent services and potential conflict of interest, at least when it comes to the presidential candidates' correspondence with federal agencies, a random sampling of which was reviewed by ABC News over the last year.
In general, most of the correspondence consists of McCain and Obama forwarding letters from constituents concerned about everything from their telephone bills and veterans' benefits to job demotions and patent disputes.
Although constituent services are considered by many congressmen to be an essential part of their duties, congressional rules limit interventions with federal agencies on behalf of constituents. Staffers can facilitate the administrative process, encourage an agency to give a case consideration and sometimes advocate for a favorable outcome. But they "cannot force an agency to expedite a case or act in favor of a constituent."
In the correspondence, many of the constituents' names have been redacted by agencies ranging from the Securities and Exchange Commission to the Pentagon due to federal privacy law so it is impossible to ascertain the results of the inquiries.
McCain, as a senior member of the Senate's Commerce Committee, has sent hundreds of letters to the Federal Communications Commission on behalf of his constituents.
Since the Arizona Senator's involvement in the Keating Five scandal, when he was criticized by the Senate Ethics Committee for "questionable conduct" for meeting with federal regulators to discuss the government's investigation of a savings and loan association owned by longtime contributor Charles Keating, McCain has generally gone out of his way to avoid the appearance of impropriety.
When it comes to constituent concerns, McCain often adds a paragraph to his letters saying that "I ask for no preferential treatment for this petitioner or any single or select group of interests, nor do I advocate a specific outcome for this or any other petitioner."
Mark A. Voigt, a former vice president of Keating's company American Continental Corporation, is a longtime friend of McCain and the current president of the board of directors of the largest private non-profit school in Arizona for students with disabilities.
When he asked for McCain's help to speed up the FCC's decision on a grant to fund the school's computer equipment in July 2005, Voigt wasn't concerned that his association with Keating made him persona non grata with the senator.
"He was always ethical with me. He knows who I am but there was no problem whatsoever," Voigt told ABC News. "There is a difference between going to bat on behalf of a huge corporation and helping some private non-profit agency. He was very helpful – John's a friend of mine. As far as I know, we got the grant taken care of."
Voigt says he never personally spoke with the Senator about the issue and only dealt with his staffers.
And though Voigt is supporting McCain for president, he has not contributed any money to the candidate in over two decades. "Since the Keating days, I try to lay low."
With other old friends, McCain has sometimes been a little more persistent in his correspondence with federal agencies but seems careful not to push it too far.
Maurice Coburn, a radio station owner in Arizona whose wife was the Lake Havasu City campaign manager for McCain's first run for Senate, wrote numerous letters to McCain, asking him to speak to the FCC chairman about getting the agency to reverse the forfeiture of the broadcast license for one of his stations.
McCain wrote seven letters over four years to the agency, inquiring about the status of the decision, though broadcast lawyers say the agency has almost never reversed such a decision. In the end, Coburn's request was denied and he is currently suing the FCC in federal appeals court.
"I didn't expect a political decision," says Coburn, who also appealed to Senator John Kyl. "They all were afraid of being accused of political interference… we did not get or expect special treatment."
When Marcus Lamb, the owner of Daystar Television, the second-largest Christian TV network in the world, wrote to McCain in March 2006 to complain that his network was bumped off the air in Phoenix by a cable company, he pulled out all the stops.
"My wife, Joni, and I are registered Republicans," noted Lamb, adding, "Rest assured that your help will be remembered and appreciated. If there is ever anything that I can do for you, please don't hesitate to let me know."
McCain wrote the agency, forwarding Lamb's letter and including his standard disclaimer about not seeking preferential treatment. Lamb's network went back on the air.
In that case and another letter on behalf of American Media Services, McCain was lightly scolded by the agency's counsel for violating ex-parte rules, which require correspondence in restricted proceedings to be sent to all the parties involved.
According to FCC lawyers, they send letters to members of Congress several times a month about such oversights, which are not considered serious.
Earlier this year, the New York Times reported that McCain wrote two letters to the FCC in 1999 on behalf of Paxson Communications, which wanted to purchase a Pittsburgh TV station and whose founder had contributed $20,000 to his 2000 presidential campaign. Then-FCC chair William Kennard described the forceful tone of McCain's letters as "highly unusual," according to Newsweek.
In a 2002 deposition cited by Newsweek, McCain said: "My job as chairman of the committee … is to see that bureaucracies do function. Bureaucracies are notorious for not functioning and not making decisions. I believe that Mr. Paxson had a legitimate complaint. Not about whether the commission acted favorably or unfavorably, but that the commission act."
A spokesman for McCain noted that the Senator adheres to the highest ethical standards when it comes to serving his constituents, who include veterans with problems getting health benefits.
Senator Obama's correspondence history is necessarily thinner considering his briefer tenure in the Senate.
In one case, Obama requested from the Department of Labor's employment standards division an "immediate investigation into the manner in which a federal contractor in Illinois has treated a group of disabled employees, which has resulted in a significant pay cut for these employees."
An administrator for the office replied that an investigation into the matter was already underway.
Among the constituents requesting Obama's assistance are some campaign contributors who claim that they didn't expert or receive any special favors.
Several members of Bethel New Life, a faith-based community development non-profit group based in Chicago, have contributed small amounts to Obama's presidential campaign.
Last year, Obama wrote a letter to the Department of Justice, supporting Bethel New Life's successful application for a Weed and Seed grant to reduce crime and drug use in the community.
"We're a faith-based community development group and we don't have a lot of money," says Mary Nelson, the group's former president and CEO who contributed $200 to Obama's campaign. "So if we gave $5 here and $10 there, I don't think that influenced his vote."
In 2006, Obama wrote a letter on behalf of the president of Seaway Bank, the largest black-owned financial institution in the Midwest, which was requesting that the Department of Transportation's Airport Concessions Disadvantaged Business Enterprise program raise the size standards to include banks with $1 billion in assets.
Seaway's chairman Jacoby Dickens has contributed $8,600 to Obama's campaigns over the last decade and the bank's president Walter Grady contributed $1000 to his Senate campaign, according to campaign finance records.
After receiving a number of similar comments and letters, the agency increased the size standard to $750 million in April 2007.
Dickens, who says he cohosted a fundraiser for Obama in Florida last year, explains that the bank was concerned about losing the concession for foreign exchange services at O'Hare Airport.
He claims that he is always conscientious about "stepping over that line" in his dealings with Obama. And he believes that the Senator's decisions would not be swayed by his campaign contributions. "I've known this gentleman forever, since when he was a community organizer and that's not how he comes."
Sometimes, those constituents represent companies located far from Illinois.
As reported by ABC News' Justin Rood, Obama quietly worked with corporate lobbyists representing two foreign companies with offices in Illinois to help pass breaks worth $12 million. At the time, an Obama spokesman said the senator was just "help[ing] his constituents" by lowering their costs for certain imported chemicals
Employees of one of those firms, Astellas Pharma, gave $1100 to Obama's campaign last year.
A spokesman for Obama, in a statement emailed to ABC News, said that the senator "is committed to providing Illinoisans with quality constituent services, and that has been a top priority during his time in the Senate. Our office serves as a liaison for constituents and the federal government, and over the years we have assisted thousands of Illinoisans with such issues as veterans' benefits, Social Security benefits, immigration, and foreign travel. Every constituent that walks into our offices and contacts us by phone or email is directed to an appropriate staff member. Each of these requests is treated equally and fairly."
In general, it's difficult to gauge whether longtime friends or campaign contributors get special treatment due to the lack of transparency on the part of Congress, which is not subject to freedom of information laws, says Bill Allison, senior fellow at the Sunlight Foundation.
"The question is, of all the people contacting the congressional office, who gets phone calls and letters on their behalf? Are they putting more effort into a request from a contributor or a friend compared to a regular constituent?"
Based on the correspondence reviewed by ABC News, it appears that McCain and Obama did not grant special favors to constituents who were friends or campaign contributors.