WASHINGTON -- With strict new ethics rules and a clearout of top aides, Republican John McCain is battling to avoid being tainted by Democrats as a Washington insider in thrall to big business.

At least five senior staffers have parted ways with McCain's White House campaign in recent days, after the Arizona senator clamped down on lobbyist activity that might pose a conflict of interest with his presidential bid.

Two had done public relations work for the widely reviled military regime of Myanmar; another lobbied on behalf of Saudi Arabia's oil-rich sheikhs.

McCain, facing a possible general election contest with Democrat Barack Obama and his message of change, is courting independent voters with the image of a maverick unafraid to take on his own party on matters of principle.

So being tied to the ultimate insiders of Washington's "K Street" crowd, named after the locale of lobbying companies' swanky offices in the capital city, is not the posture that McCain wants to project.

A week ago, he imposed draconian rules on his staff including a ban on registered lobbyists or "foreign agents" and a demand for full disclosure of outside activity by part-time volunteers.

Obama, whose campaign banned registered lobbyists from the start, said McCain was "very much a creature of Washington" after the departure from the Republican's ranks of a key finance aide, Tom Loeffler.

The McCain camp's reaction to that slight showed the stakes at play for a candidate keen to protect his election appeal as a Washington outsider.

McCain spokesman Tucker Bounds revived allegations about Obama's Chicago ties to William Ayers, an "unrepentant domestic terrorist" once part of the 1960s radical group Weather Underground.

"If Barack Obama is going to make associations the issue, we look forward to the debate about Senator Obama's associations and what they say about his judgment and readiness to be commander-in-chief," Bounds said.

McCain has a track record of taking on vested interests: his Senate investigations helped convict one of the most powerful Republican lobbyists, Jack Abramoff, over charges of bribery and influence-peddling.

The Arizona senator is also a crusader against lobbyist-tainted money in politics, driving through a campaign finance reform in 2002 and decrying "pork-barrel" spending such as an infamous "bridge to nowhere" in Alaska.

However, there are signs of a backlash among the lobbying fraternity, who are a potent force in political fundraising.

Washington's Politico newspaper last week recounted the unhappiness of several Republican lobbyists who stood by McCain when his campaign looked down and out because of a cash crunch in mid-2006.

"McCain's self-righteous (expletive deleted by Politico) has caught up with him. Now he's got himself in a jam," one was quoted as saying. Another said: "If we're not good enough, then send my check back. It pisses me off."

Obama for his part says that both McCain and his Democratic primary opponent, Hillary Clinton, represent all that is wrong with Washington and pledges that no lobbyists will be allowed into his White House.

But the Illinois senator does permit registered lobbyists, who in Washington number more than 20,000 representing every imaginable industry and interest group, to work as unpaid volunteers and advisers.

The Republican National Committee decried that stance as "hypocrisy," accusing Obama of wanting to "do as I say, not as I do."

Jan Witold Baran, a partner at Washington law firm Wiley Rein who specializes in government ethics, said "the issue involving lobbyists undermines McCain's assertions that he is an independent maverick."

But addressing both the candidates' efforts to keep lobbyists at arm's length, Baran told AFP: "It's impossible."

"The right to petition government for redress of grievances is preserved in the first amendment of our constitution," he said, noting that Washington's biggest lobbying operation does not represent an industry, but retired people.