WASHINGTON — For Rick Noriega, tomorrow’s presidential primary in Texas may amount to a mighty shove off the political fence.

The likely Democratic nominee for Senate in Texas, Mr. Noriega has tried for months to stay neutral in the contest between senators Clinton and Obama, not wanting to risk backing the wrong horse and damaging his own candidacy. The peculiar nature of the Texas primary could force him to make his choice public.

To fully participate, Texas Democrats must go to the polls during the day and also attend an open caucus — known as a precinct convention —in the evening. While voting on Election Day is a no-brainer for most politicians, the question surrounding Mr. Noriega’s campaign in recent days has been: Come Tuesday night, will he show up to caucus?

A state representative who served with the Army in Afghanistan, Mr. Noriega has three opponents of his own in the Democratic primary Tuesday, but he is heavily favored to win the right to challenge the incumbent Republican, Senator Cornyn, in the fall.

Mr. Noriega plans to attend his precinct convention, according to a public schedule released by his campaign, but he appears intent on staying mum about his choice until the last possible moment. “Rick is not publicly endorsing, but he’s looking forward to working with the Democratic nominee,” a campaign spokeswoman, Holly Shulman, said in a statement yesterday.

By not announcing his decision beforehand, Mr. Noriega avoids the possibility of alienating supporters of either Mrs. Clinton or Mr. Obama who may be undecided about his own candidacy. If he skipped the caucus entirely, he could open himself up to criticism that he was setting a poor example in civic participation.

A spokesman for Mr. Cornyn declined to comment on Mr. Noriega’s conundrum. Mr. Cornyn has endorsed Senator McCain for president.