Ahead of Tuesday's primaries, Ohio's Cleveland Plain Dealer newspaper found Senator Clinton slightly ahead in the central US state, by 47-43 per cent.

Senator Obama led in Texas by 46-45 per cent, according to polling by McClatchy Newspapers, MSNBC television and the Fort Worth Star-Telegram.

The results were within the four-point margin of error for both polls.

Almost one voter in 10 remains undecided in each state, more than enough to swing the contests in either direction.

Ohio and Texas are the main prizes in Tuesday's voting. The northeastern US states of Rhode Island and Vermont are also holding primaries.

With Obama basking in 11 nominating wins in a row, Clinton is likely to face intense pressure to fold her White House campaign should she lose both Ohio and Texas. -- AFP