WASHINGTON -- Republican George W. Bush has opened up a four-point advantage over Democrat Al Gore, according to the Reuters/MSNBC daily tracking poll released Sunday.

Sixteen days before the Nov. 7 election, the race stands at 45 percent for the Texas governor and 41 percent for the vice president in the poll of 1,206 likely voters conducted Thursday through Saturday by pollster John Zogby. This is the largest Bush lead since the daily survey began on Sept. 29.

The results remain within the margin of error of plus or minus three points. However, Gore's support slipped by three points in one day, suggesting that Bush may be tightening his grip in one of the closest presidential races in decades.

Green Party candidate Ralph Nader polled 5 percent; Reform Party hopeful Pat Buchanan scored 1 percent, while 7 percent of voters remained undecided.

The poll's sampling margin of error means that either candidate's support may be 3 points more or less than the survey's result on any given day. Therefore, a candidate must have a lead of greater than 6 percentage points to state definitively that he is ahead.

"Gore has a problem. In addition to not getting his message through, the difference in this race right now is Ralph Nader," said Zogby.

"If Gore moves to the left with a populist message, he risks losing ground in the vital center. If he moves to the center he will watch Nader's support increase," he said.

Bush is now leading considerably among 18- to 24-year olds, who have been the most volatile sector of the electorate. But the Texan also leads among suburban voters, parents and married voters. The two candidates are tied now among those who say they live from paycheck to paycheck. Gore at one time had a huge lead among this group.

In the equally close race for control of the House of Representatives, where Democrats need a net pickup of only seven seats to regain the majority, Republicans have a two-point lead.

In the presidential race, 86 percent said they were now unlikely to change their votes.

Forty percent said they had definitely ruled out voting for Bush but 43 percent said that about Gore.

Gore led Bush on both the East and the West coasts. Bush was far ahead in the South but most significantly had opened a 13-point margin in the crucial Midwest region, where many experts believe the election will be decided.

While Bush is winning the support of more than 85 percent of Republicans, Gore only has the backing of 75 percent of Democrats. Some 13 percent of Democrats are now backing the Texas governor.

Bush leads among white voters by 51 percent to 36 percent and is taking 35 percent of the Hispanic vote. While Bush leads among men by 14 percentage points, Gore's lead among women is only four points.

Reuters and MSNBC will release a new poll every day until the election.

Since the poll began on Sept. 29, the race has never been outside the survey's statistical margin of error of plus or minus three points.