On the eve of a heated Democratic runoff for the U.S. Senate, political heavyweight Ron Kirk, who is black, won an important Hispanic endorsement over Mexican-American schoolteacher Victor Morales.

Democratic nominee for governor Tony Sanchez -- whose spokesman said last week that Sanchez did not plan to endorse either candidate in the Senate race -- threw his support behind Kirk. Sanchez, like Morales, is Mexican-American.

"It has grown apparent, especially in the last few days of this campaign, that Ron Kirk has earned this party's nomination and we're going to do what we can to help him," said Sanchez spokesman Mark Sanders.

Kirk is the first black mayor of Dallas; Morales became a political celebrity by campaigning for the Senate in 1996 in his pickup truck.

Regardless of the outcome of Tuesday's runoff, a minority candidate will win the nomination to battle the Republican contender in a deeply conservative state that has never had a black or Hispanic senator.

Kirk spent Monday campaigning across the state, while Morales taught high school geography.

The difference between the two candidates has been ever-present in the tense runoff, which pits Kirk, the polished, well-funded party favorite, against Morales, the underfunded, pickup-driving underdog.

Still, a Dallas Morning News poll released Saturday showed Morales leading Kirk among likely Democratic voters, 44 percent to 37 percent. The telephone poll of 625 voters was conducted Monday through Thursday and had a margin of error of plus or minus 4 percentage points.

Either Democrat would face an uphill battle in a state that has not had a Democratic senator in nearly a decade. The GOP nominee, Texas Attorney General John Cornyn, is anointed by President Bush and has a hefty war chest.

At stake is the seat held by Republican Phil Gramm, who is retiring. Since the Democrats hold a one-seat edge, every close Senate race this year is potentially pivotal.

Kirk touting is touting a bipartisan, bridge-building record at Dallas City Hall, and Morales is pushing his ability to relate to average folks.

Morales draws the bulk of his support from South Texas, with its large Hispanic population. He is expected to benefit from high turnout in the Rio Grande Valley.