D.A. Weibring matched McNulty's 65 to move into a tie for second place at 9-under-par 133. He is joined there by Larry Nelson, who posted a 2-under 69. Vicente Fernandez, Tom Jenkins and Tom Purtzer each carded 4-under rounds of 67 to share fourth place with Mike McCullough (70) at minus-8.
McNulty, who is looking for his first victory on United States soil, started the day four shots behind Tom Kite, the overnight leader. McNulty, though, came out firing. He rolled in a birdie at the 10th, his first, and came back with another birdie on No. 12 at TPC of Tampa Bay.
He continued to make up ground with his third birdie of the day at the par-5 14th. He dropped a sand-wedge within six feet to set up birdie at the 16th. The 50-year-old then drained another birdie on No. 18 to make the turn at 9 under with a share of the lead.
McNulty, the medalist at the 2003 Champions Tour National Qualifying Tournament, stumbled to his lone bogey of the day at the first. However, he erased that mistake with a three-foot birdie putt at the fifth to regain a share of the lead.
The Zimbabwe native grabbed the overnight lead when he two-putted for birdie at the par-5 seventh to climb to minus-10.
'I'm excited. I've looked forward to playing the Champions Tour for about a year and a half,' said McNulty. 'I've always been a good wind player.'
McNulty owns 55 wins worldwide, but has never won in the United States. He is looking to become the 12th player in Champions Tour history to win in his first event on tour.
'Most guys on the Champions Tour are champions in the past,' McNulty said. 'Playing in Europe the last few years, I didn't have much in common with the young guys.
'Not many guys compete on the regular tour right up until they're 50. I've taken that experience over to the Champions Tour and I hope to be in position to win several times. You can't buy experience.'
Weibring opened with a birdie at the first and converted another at the par-5 seventh. Around the turn, he ran off three straight birdies from the 12th to climb to 8 under. He capped his day with a birdie on the 16th.
'I hit lots of good shots today,' said Weibring. 'Today I scored much better. I feel good about my game.'
Nelson picked up his first birdie at the par-5 12th. He later faltered to consecutive bogeys from the 17th to fall to minus-6. He then birdied the second and fifth. Nelson's birdie at the par-5 seventh gave him the lead at minus-9 before he was joined there by Weibring and passed by McNulty.
'It was a perfect morning,' Nelson said. 'No excuses for not playing as well. I was a little more cautious and didn't hit the ball as well as I did Friday.'
Dana Quigley and Gil Morgan posted matching rounds of 2-under 69. They share eighth place at 7-under-par 135. John Bland and Graham Marsh are one stroke further back at minus-6.
Kite was looking to halt a streak that has seen no first-round leader or co-leader go on to win the tournament since the event shifted to TPC of Tampa Bay. He stumbled to a 3-over 74 and is joined at 5-under-par 137 by Jim Thorpe (67) and David Eger (72).
Bruce Fleisher, the 2003 champion, struggled to a 3-over 74. He stands at 1-under-par 141.