The races for both the Democratic and Republican US presidential nominations could be settled as four states vote in a major round of primary elections, BBC reported Tuesday.

The main focus is on the polls in Ohio and Texas but votes are also taking place in Rhode Island and Vermont.

Democratic front-runner Barack Obama has said rival Hillary Clinton may have to quit if he wins in Texas and Ohio, but she has vowed to stay the course.

John McCain could be confirmed as the Republican candidate in the polls as he is expected to beat his rival Mike Huckabee in all four states.

Victory could secure him enough of the 256 delegates at stake to clinch his party's nomination at its national convention in September.

For the Democrats, opinion polls suggest Obama has a narrow lead over Clinton in Texas, but that she is clinging on to her modest lead in Ohio.

The former first lady has played down suggestions that Tuesday's results could prove a make-or-break moment.

Her last win in a primary or a caucus was during the nationwide Super Tuesday contests on February 5, since when Obama has been on an 11-contest winning spree.

During weekend rallies in Ohio, Hillary Clinton focused her attacks on her opponent's foreign policy and national security experience.

For the Democrats, a total of 370 delegates is at stake in Tuesday's four races, most of them in Texas and Ohio.

The Illinois senator currently has 1,385 delegates to Clinton's 1,276. A total of 2,025 is needed to secure the Democratic Party's nomination at its national convention in August.

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