Houston: Barack Obama holds a slight lead on Hillary Clinton in Texas and has almost pulled even in Ohio before contests that could decide their US Democratic presidential battle, according to a Reuters/C-SPAN/Houston Chronicle poll released on Friday.

The contests on Tuesday are crucial for Clinton fighting to halt Obama's streak of 11 consecutive victories in their battle for the Democratic nomination for the November 4 presidential election.

Obama, an Illinois senator, has a 6-point edge on Clinton in Texas, 48 per cent to 42 per cent. He trails Clinton 44 per cent to 42 per cent in Ohio - well within the poll's margin of error of 3.8 percentage points.

In the Republican race, front-runner John McCain holds commanding leads over his last major rival, former Arkansas Gov. Mike Huckabee.

McCain, an Arizona senator, has built an unassailable advantage in delegates who will pick the nominee at the Republican Party convention in September.

The poll, conducted by Zogby International, found McCain with big double-digit margins over Huckabee in Texas and Ohio. Among Democrats, Obama has a big edge with voters in both states who made their decision within the last month.

Clinton led comfortably in both states among voters who decided more than a month ago.

Other opinion polls show tightening races in both states, where Clinton enjoyed big leads just a few weeks ago. "All the momentum is clearly with Obama," pollster John Zogby said.