John McCain officially becomes the Republican presumptive nominee as the Republican National Committee confirms he has now won enough delegates to seal the nomination. President Bush endorses him. In the Democratic race, the Obama campaign ratchets up its attacks on Hillary Clinton's personal finances and on her claim to have foreign affairs experience, while Hillary Clinton hints that if she won the race she would consider picking Mr Obama as her running mate.

"He is going to be a president who will bring determination to defeat the enemy and a heart big enough to love those who hurt."
George W Bush endorses John McCain

It's cool to believe in Obama now, but it may become cool to see through him by the fall Ramesh Ponnuru National Review At-a-glance: 4 March

"That may be where this is headed, but of course we have to decide who is on the top of the ticket."
Hillary Clinton does not rule out running on a joint ticket with Barack Obama

"We are just focused on winning the nomination. That is my focus. I respect Sen Clinton. She has been a tenacious opponent. It is premature to talk about a joint ticket."
Barack Obama weighs in on the joint ticket question

"The default option in our culture is scepticism, even delegitimisation. It's cool to believe in Obama now, but it may become cool to see through him by the fall."
Ramesh Ponnuru, National Review

In the run-up to the 4 March primaries, conservative radio host Rush Limbaugh argued that Republicans who wanted to prolong the increasingly divisive Democratic race should vote for Hillary Clinton in the Ohio and Texas contests.

Emails to the conservative National Review website provided anecdotal evidence that some Republicans did indeed follow Mr Limbaugh's advice.

David Weigel, who blogs at Reason.com, attempted to determine the extent to which these "cross-over Republicans" were a factor in Hillary Clinton's 4 March victories, by comparing exit poll data from the Wisconsin primary with findings from Ohio and Texas.

He found that - in Wisconsin - Barack Obama won 72% of the self-declared Republicans who voted in the Democratic primary, while 28% of them chose Mrs Clinton.

In Ohio, however, Republicans voting in the Democratic primary were split 49-49 between Mr Obama and Mrs Clinton, while in Texas, Mr Obama only won the Republican vote 52%-47%.

Mr Weigel noticed similar results when he compared the votes of self-described "conservatives" in Democratic contests in Wisconsin, Ohio and Texas.

So the exit polls suggest either that Mrs Clinton was able to win over large numbers of Republicans in Ohio and Texas - or that some Republicans were deliberately attempting to keep Mrs Clinton in the race in order to exacerbate divisions within the Democratic Party.

Could George W Bush's current unpopularity prove to be a handicap for John McCain in the campaign?