Barack Obama is expected to hit back aggressively against John McCain on Monday after the Republican gave the go-ahead to a fresh wave of attacks against the Illinois senator’s character and patriotism as the Democrat consolidates his lead in the presidential race.

A new Obama TV advertisement, previewed over the weekend, will brand Mr McCain as “erratic in a crisis” and accuse him of “dishonourable” smear tactics aimed at distracting from the financial crisis.