Obama accuses Republicans of "feeding" Trump base for years

US president Barack Obama has accused Republican lawmakers of weak leadership, and suggested they now cannot control the state of their party as Donald Trump's campaign to become the next occupant of the White House seems to be imploding.

Trump's controversial policies and frequent outbursts have made him a unique candidate for the GOP, and Obama said that the party has gone too far.

The president was speaking to 2,000 people at the Ohio Democratic Party's fundraising dinner in Columbus on Thursday evening.

"The problem is not that all Republicans think the way this guy does," Obama told the dinner. "The problem is they've been riding this tiger for a long time. They've been feeding their base all kinds of crazy for years."

Last week, a leaked tape emerged from 2005 in which Trump boasted of sexual misconduct to Access Hollywood reporter Billy Bush, and since then several women have come forward to accuse the Republican candidate of inappropriate conduct.

A plethora of high-profile members of the GOP have since withdrawn their support from Trump.

"They don't get credit for at the very last minute when finally the guy that they nominated and they endorsed and they supported is caught on tape saying things that no decent person would even think, much less say, much less brag about, much less laugh about or joke about, much less act on," Obama said. "You can't wait till that happens then say it's too much."

Clinton and Trump are engaged in a tense battle in Ohio, and it is likely to be one of the key states in the November 8 election. There are also tight races in Florida and Georgia.