Thousands of disenchanted Americans are taking part in what is known as vote trading in a tactical effort to keep Donald Trump out of the White House.

Key points: Republicans are legally trading votes between electoral boundaries

Republicans are legally trading votes between electoral boundaries They are aiming to stack pro-Clinton votes in swing states

They are aiming to stack pro-Clinton votes in swing states Former Reagan staffer Ken Adelman is among those reluctantly voting Clinton

For example, a Republican who was against Mr Trump and intended to vote for a third party candidate instead, and also lived in a swing state, could be matched with a Clinton supporter in a safe Democratic state.

Each of these voters would agree to vote for the other's chosen candidate: the Republican who might have "wasted" their vote on a third party (where it would not be enough to stop Mr Trump) votes for Hillary Clinton while the Clinton supporter would cast in favour of the third party candidate.

So, in a bipartisan fashion, the two voters are able to deliver more votes to Mrs Clinton in a swing state where it matters more.

Using apps or websites such as #NeverTrump or TrumpTraders.org, voters in safe seats connect with voters in swing states and swap their votes legally.

The electoral effect would have more impact in the battleground states, where an extra Clinton vote could make a difference.

Co-founder of Trump Traders John Stubbs said more than 15,000 people had enrolled.

The group is targeting five swing states of Florida, North Carolina, Ohio, Pennsylvania and Texas.

"By voting tactically in 2016, conscientious Republicans can still prevent the worst-case scenario: Mr Trump taking over the White House and the GOP," Mr Stubbs wrote recently.

He also helped set up Republicans for Clinton, a group encouraging Republicans to vote for the Democrat Party nominee.

Its mission statement reads: "The threat posed by Donald Trump compels us to consider what many of us never have: supporting the Democratic nominee for President".

'It raised my blood pressure'

Long-time Republican supporter and former Reagan administration official Ken Adelman cannot stomach a Trump presidency.

"It raises my blood pressure by at least 20 points," he told Lateline.

He has endorsed the Republicans for Clinton campaign.

"Donald Trump is not a Republican. He's not a Conservative, he doesn't believe in the values that I believe in," he said.

Ken Adelman is equally dismayed by Hillary Clinton but he reluctantly voted for the Democratic nominee.

"That hurt," he said of casting his vote.

"But the fact is I'm very comfortable with that decision. Donald Trump would be a disaster as president."