The four candidates left in the Republican race will spend much of the next week criss-crossing America.
Former speaker Newt Gingrich and the libertarian-leaning Ron Paul largely skipped Michigan and Arizona to focus on Super Tuesday. A convincing performance then would put Mr Romney in the lead for the White House nod.
Supporters will be hoping his campaign’s deeper pockets should be able to bankroll the advertising spend needed if he is to be competitive in all of next week’s primary voting states.
Mr Santorum is placing his hopes in an Ohio win to rejuvenate his campaign. Meanwhile Mr Gingrich is focusing on Georgia and Tennessee in a bid to repeat a success in South Carolina.
It takes 1,144 delegates to win the nomination at the national convention in Tampa, Florida, in late August.Romney has 163. Santorum, 83, Gingrich and Paul, 19. But there are 40 delegates at stake in the Washington state caucuses this Saturday, followed by 419 on Super Tuesday.