SAN ANTONIO (Reuters) - Democrats Barack Obama and Hillary Clinton are deadlocked in Texas and Ohio heading into potentially decisive presidential showdowns, according to a Reuters/C-SPAN/Houston Chronicle poll released on Saturday.

Clinton is fighting to save her White House candidacy in the two primaries on Tuesday. Obama, an Illinois senator, has beaten her in 11 consecutive contests to take control of the race for the Democratic nomination in November’s presidential election.

In Texas, Obama leads 45 percent to 43 percent, down from a 6-point advantage on Friday and well within the poll’s margin of error of 4 percentage points.

In Ohio, they are dead even at 45 percent. Clinton, a New York senator and former first lady, had led by 2 points on Friday in the rolling poll conducted by Zogby International.

Republican front-runner John McCain had huge leads in both states over his last major rival, former Arkansas Gov. Mike Huckabee, as he moves closer to clinching the Republican nomination.

McCain, an Arizona senator, can come close to winning the 1,191 delegates needed to clinch the nomination with big wins on Tuesday. The delegates select the nominee at September’s Republican Party convention.

In the Democratic race, Clinton made overnight gains in Texas in some of her strongest voting blocs, including older and women voters. She pulled almost even with Obama among voters who made their decision in the last few days.

“Hillary Clinton may be making a connection with Democratic voters in Texas, especially among those in key demographic groups that have supported her all year,” pollster John Zogby said.

Clinton also kept her big lead among Hispanic voters in Texas, who could account for one-third or more of the total turnout. She had huge advantages in the heavily Hispanic southern and western parts of the state.

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Other opinion polls show tightening Democratic races in both states, where Clinton enjoyed big leads just a few weeks ago.

STILL UNDECIDED

In Ohio, 6 percent of Democrats say they are still uncertain of their choice. In Texas, 7 percent of Democrats are not sure, leaving plenty of room for late swings.

Clinton, who has stressed her prescriptions for the ailing economy in blue-collar Ohio, showed strength in sparsely populated and less affluent southern Ohio, where she spent two days this week campaigning.

But she trailed Obama in heavily Democratic northeast Ohio and the growing central Ohio region.

Among Republicans, McCain leads Huckabee 58 percent to 23 percent in Ohio and 54 percent to 31 percent in Texas. The other remaining Republican candidate, Texas Rep. Ron Paul, had 7 percent in Texas and 8 percent in Ohio.

The rolling poll was conducted Wednesday through Friday, with most of the survey coming before Clinton launched a fresh attack on Friday on Obama’s ability and experience handling a national security crisis.

She released a new television ad to be aired in Texas showing children sleeping peacefully in bed.

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“It’s 3 a.m. and your children are safe and asleep. But there’s a phone in the White House and it’s ringing. Something’s happening in the world. Your vote will decide who answers that call,” the ad’s narrator says

Obama hit back with an ad of his own, which also showed sleeping children and spoke of a White House phone ringing.

“When that call gets answered, shouldn’t the president be the one -- the only one -- who had judgment and courage to oppose the Iraq war from the start?” the narrator says.

The poll of 701 likely Democratic voters in Ohio and 708 in Texas had a margin of error of about 4 percentage points. The poll of 600 likely Republican voters in Ohio and 596 voters in Texas had a margin of error in both states of 4 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.

(Editing by John O’Callaghan)