In the latest CNN/Opinion Research Corp. survey, 50 percent of likely Democratic primary voters said they support Clinton for the party's nominee, with 48 percent said they back Obama.

The Texas primary, with its 193 delegates, is March 4, as are primaries in Ohio, with 141 delegates; Rhode Island, with 21 delegates, and Vermont, with 15 delegates.

The survey indicated Arizona Sen. John McCain is the clear favorite for the Republican presidential nomination. Among Republicans, 55 percent of likely Texas GOP primary voters favor McCain, while 32 percent back former Arkansas Gov. Mike Huckabee and 11 percent support home-state congressman Ron Paul. The poll's sampling error for Republican respondents is 4 percentage points.

The CNN/Opinion Research Corp. poll was conducted Friday through Sunday. Surveyors talked to 1,506 adults in Texas, including 553 likely Republican primary voters and 529 likely Democratic primary voters. For the Democratic survey, the poll has 4 1/2 percentage point plus-or-minus sampling error; the plus-or-minus error rate was 4 percentage points in the Republican race.