RICHMOND -- Republican incumbent George Allen and Democratic challenger James Webb are running neck-and-neck as they near the finish line of a bitterly contested U.S. Senate race, according to a final pre-election poll conducted for The Roanoke Times and other Virginia newspapers.

The survey by Mason-Dixon Polling & Research also shows fading support for a state constitutional amendment banning same-sex marriages and civil unions, with fewer than half of the registered voters in the poll saying they back the measure. But supporters of the amendment still outnumber opponents by a slight margin.

Webb, who trailed Allen by 16 points in a July survey, now holds a statistically insignificant lead of 1 percentage point as the candidates barnstorm the state in advance of Tuesday's election. In the poll, 46 percent favor Webb, 45 percent support Allen and 2 percent back independent Gail Parker. The remaining 7 percent are undecided.

The results come from a telephone survey of 625 registered voters conducted Wednesday through Friday. All indicated they are likely to vote Tuesday. The survey has a margin of error of plus or minus 4 percentage points.

Webb holds a commanding lead in vote-rich Northern Virginia and also has an edge in the key battleground of Hampton Roads. Allen continues to run strong in the Shenandoah Valley and much of central and Southside Virginia. Southwest Virginia has become the most competitive region of the state, according to the poll, with Allen holding a lead of just 4 percentage points. Webb is spending most of this weekend in the region, and Allen made Bristol the last stop on his busy Saturday schedule. Both candidates will visit Roanoke on Monday.

The Virginia contest is one of a handful of key races that will determine whether Republicans keep control of the Senate. Democrats must gain six seats in the 100-member chamber to wrest control from the GOP, a scenario that once seemed implausible.

Few had expected the Virginia race to be close when the campaign began. But Webb, a former Navy secretary and author, appears to have gained from public discontent with President Bush and Congress, growing opposition to the war in Iraq, and Allen's campaign missteps.

Webb worked in the Reagan administration and endorsed Allen in 2000, but has been an outspoken critic of the Iraq war and the Bush administration's handling of national security issues. He also blasts the GOP for fostering an economy in which corporate profits have soared at a far greater rate than wages and salaries.

Allen has promoted his achievements as Virginia's governor in the mid-1990s and his stands on issues such as tax relief, immigration, energy independence and combating terrorism.

Allen had long been a steadfast supporter of Bush's Iraq policy. But he edged away from the president last month after fellow Republican John Warner, Virginia's senior senator, returned from Iraq and grimly declared that the country was "drifting."

The Mason-Dixon poll contains some troubling signs for Allen, who had been considered a promising candidate for the GOP's 2008 presidential nomination:

n When asked which party they would prefer to have control of the Senate, 47 percent chose Democrats and 45 percent chose Republicans.

n Allen has not reached the 50 percent mark in any of five Mason-Dixon polls conducted since July. His unfavorable rating has reached an all-time high of 42 percent and has nearly doubled in a span of just four months.

n Black voters favor Webb by a margin of 87 percent to 6 percent, a possible result of campaign controversies that raised questions about Allen's attitudes on race. Brad Coker, the director of the Mason-Dixon poll, noted that Allen typically attracts between 12 percent and 15 percent of the black vote.

n A plurality of voters said Webb would do a better job than Allen on a host of issues, including the Iraq war and national security, health care, education and government ethics. Webb even holds a 7-point edge over Allen on moral issues and family values, typically a strong suit for Republican candidates in Virginia.

The only issue on which Allen holds a clear advantage is immigration. The senator has taken a hard-line stand against any sort of amnesty for illegal immigrants already in the country, distancing himself from a more flexible policy advocated by Bush.

Allen also has tried to rally his conservative base by trumpeting his support for a state constitutional amendment banning same-sex marriages and civil unions. Allen, like other Republican statewide officials, insists that the amendment is needed to safeguard Virginia's prohibition on same-sex unions from court challenges.

But support for the amendment is slipping, according to the poll. Just 49 percent said they favor the measure, and 45 percent said they oppose it. Support for the amendment remains strong in Southwest Virginia, where nearly two-thirds of the voters favor it. Opposition is strongest in Northern Virginia, where 60 percent plan to vote against the measure.

Nineteen states have passed similar amendments since 1998, and no state has had an amendment rejected by voters. Virginia is one of eight states with marriage amendments on their ballots Tuesday.

Webb has joined high-profile Democrats such as Gov. Tim Kaine and former Gov. Mark Warner in opposing the amendment, arguing that it contains ambiguous language that could threaten certain legal arrangements between unmarried individuals.

Neither Allen nor Webb appears to have gained from a heavy volume of negative advertising, much of it funded by national party organizations. Nearly half the voters described the tone of the campaign as "very negative," and 59 percent said both candidates have been attacked unfairly. When asked which candidate had waged the more negative campaign, 32 percent chose Allen and 19 percent chose Webb. But 45 percent said the candidates have been equally negative.