At the same time, Bush will discuss at a White House event his efforts to encourage work that could make additional stem cell lines available for research, presidential spokesman Tony Fratto said Tuesday.

The president has accused majority Democrats of recycling an old measure that he already vetoed and argued that the bill would mean American taxpayers would _ for the first time _ be compelled to support the deliberate destruction of human embryos.

"The president supports and encourages stem cell research _ including using embryonic lines _ as long as it does not involve creating, harming or destroying embryos," Fratto said. "That is an ethical line that should not be crossed."

Democrats made the legislation a top priority when they took control of the House and Senate in January, but they don't have enough votes to override Bush's decision.

Senate Majority Leader Harry Reid appealed to Bush on Tuesday not to veto the bill. He said the measure acknowledges the ethical issues at stake and offers even stronger research guidelines than exist under the president's current policy.

House Speaker Nancy Pelosi used Bush's veto threat as a reason to send out an e-mail letter soliciting contributions to the Democratic Congressional Campaign Committee to help elect more Democrats.

"By vetoing a bill that expands stem cell research, the president will say `no' to the more than 70 percent of Americans who support it, `no' to our Democratic Congress' fight for progress, and `no' to saving lives and to potential cures for diseases such as diabetes and Parkinson's," Pelosi wrote. "He will say `no' to hope."

In light of the veto, Sen. Norm Coleman, R-Minn., who planned to be at the White House event, sought support for a stem cell bill he is sponsoring. It has passed the Senate but has not yet been taken up by the House.

"My stem cell bill, which passed the Senate with broad bipartisan support, offers a clear alternative for our colleagues in the House to significantly expand federally funded stem cell research, while ensuring no taxpayer dollars are used for the destruction of human embryos," Coleman said.

Coleman urged Democrats who favored the bill Bush was to veto to get behind his legislation.

"Those who support the stem cell research bill ... are at a definitive crossroads," he said. "Do they seek to advance lifesaving research for millions of Americans suffering from serious disease or do they, in fact, prefer to keep stem cell research at a political stalemate? "

This will be the third veto of Bush's presidency. His first occurred last year when he rejected legislation to allow funding of additional lines of embryonic stem cells _ a measure that passed over the objections of Republicans then in control. Earlier this year, he vetoed legislation that would have set timetables for U.S. troop withdrawals from Iraq.

Opponents of the latest stem cell measure insisted that the use of embryonic stem cells was the wrong approach on moral grounds _ and possibly not even the most promising one scientifically. They cite breakthroughs involving medical research conducted with adult stem cells, umbilical cord blood and amniotic fluid, none of which involve the destruction of a human embryo.

The science aside, the issue has weighty political implications.

Public opinion polls show strong support for the research, and it could return as an issue in the 2008 elections.

Democratic presidential candidate Hillary Rodham Clinton appeared in Hanover, N.H., this week with a child who has diabetes and a paralyzed 23-year-old to urge Bush not to veto the bill. Last month, the issue was a topic at a debate with Republican presidential hopefuls in California.

The bill Bush is vetoing passed Congress on June 7, drawing the support of 210 House Democrats and 37 Republicans. That was 35 votes fewer than needed to override a veto. The Senate cleared the bill earlier by a margin that was one vote shy of the two-thirds needed to overcome Bush's objections.

According to the National Institutes of Health Web site, scientists were first able to conduct research with embryonic stem cells in 1998. There were no federal funds for the work until Bush announced on Aug. 9, 2001, that his administration would make the funds available for lines of cells that already were in existence.

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