Presumptive Republican presidential candidate John McCain's campaign suffered another embarrassment when one of his key fundraisers resigned, becoming the fifth person to leave because of ties with lobbying firms.

McCain's national finance co-chairman Thomas Loeffler also became the highest placed official in the campaign to resign.

The former tax representative's stepping down comes in the wake of Democratic front-runner for presidential nomination Barack Obama vowing to work for ending the influence of lobbyists in Washington.

Obama, analysts note, has not accepted money from the lobbyists, but the top corporate bosses have contributed to his campaign and his opponents point out that the bosses employ lobbyists.

However, the Democrat took a swipe at McCain, saying that it shows that he is very much a "creature of Washington."

He went on to repeat that the country cannot afford to have vested interests dictate policies and promised to change the way Washington works.

"It does appear that over the last several weeks that John McCain keeps on having trouble with his top advisers being lobbyists either for foreign governments or big business," he said.

But McCain's campaign officials threatened that if Obama rakes up the issue of connections, they could respond in kind.

Meanwhile, both Obama and his party rival Hillary Clinton were campaigning hard in Oregon and Kentucky where the primary is due on Tuesday.

Polls show that Clinton is leading in Kentucky, but Obama is ahead in Oregon.

Some media reports said the campaigns of Obama and Clinton are already in touch to work together after the issue of nomination is decided.

In television interviews Clinton pledged to go to the finish, but Obama was already joining issues with McCain and referring to Clinton in the passing.

While Clinton was campaigning only in the states where the primaries are due, Obama campaign was announcing his plans to visit the states where he lost primaries to drum up support.

It is important that he wins over Clinton's voters if he finally emerges as the candidate as a large percentage of her supporters have said in the opinion polls that they would either not vote or vote for McCain.

Obama is also strengthening staff in swing states to strengthen the Democratic base and ensuring that those who support him register themselves to vote.