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

The contests on Tuesday are crucial for Clinton, a New York senator and former first lady 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 percent to 42 percent. He trails Clinton 44 percent to 42 percent 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. The clearest indicator is the line of demarcation between those who decided early and those who are deciding late. The question is whether she can stem the tide.

In Ohio, 9 percent of Democrats said they were still uncertain of their vote. In Texas, 7 percent of Democrats were not yet sure, leaving plenty of room for late swings.

CLINTON'S BASE OF SUPPORT

Clinton's slight advantage in Ohio was built among some of her core constituencies, including women, older voters, Democrats, Catholics, union households and voters outside the state's three biggest cities.

Obama, who would be the first black U.S. president, leads in Ohio among independents, young voters, higher-income voters and blacks.

In Texas, the two are essentially tied among Democrats, while Clinton has big leads in the heavily Hispanic southern and western portions of the state.

Clinton, who would be the first woman president, has a double-digit advantage among Hispanics in Texas. They could account for one-third or more of the state's primary voters.

The question in Texas is who turns out to vote, and how big is the Hispanic turnout, Zogby said.

Among Republicans, McCain leads Huckabee 62 percent to 19 percent in Ohio and 53 percent to 27 percent in Texas. The other remaining candidate, Texas Rep. Ron Paul, had 11 percent in Texas and 8 percent in Ohio.

McCain could come close to clinching the nomination with big wins in the two states. Vermont and Rhode Island also vote on Tuesday.

The rolling poll was conducted Tuesday through Thursday, with most of the survey coming after Tuesday night's combative debate in Ohio between the two Democrats that featured a series of sharp exchanges on health care, trade and Iraq.

Clinton returned to Texas on Thursday night after announcing she had raised $35 million in February, her biggest month of fundraising. That gives her the resources to continue the nominating fight if she can pull out wins on Tuesday.

The poll of 708 likely Democratic voters in Ohio and 704 in Texas had a margin of error in both states of 3.8 percentage points. The poll of 592 likely Republican voters in Ohio and 605 voters in Texas had a margin of error in both states of 4.1 percent.

In a rolling poll, the most recent day's results are added and the oldest day's results are dropped to track changing momentum. The poll will continue until Tuesday.

Arizona New York Barack obama Hillary Clinton