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With four rounds of fixtures to go, the Crues are now on the verge of securing back-to-back league titles for the first time in the club's history.
Whyte rounded Ross Glendinning and flicked the ball home on 88 minutes.
Substitute Forsythe fired the second into the top corner in stoppage time.
Linfield went into the crucial top-of-the-table clash on the back of a formidable record of just one defeat in their previous 15 Premiership fixtures.
However, Crusaders had suffered only three reverses in the last 16 Premiership meetings between the sides.
Goalscoring chances were at a premium in a cagey first half but Ross Glendinning denied Paul Heatley and Jordan Owens fired wide of the post for the hosts.
Linfield were the superior team for much of the second period and Sean O'Neill pushed a fine Stephen Lowry drive onto the post.
Glendinning produced another fine save as Heatley attempted to find the top corner, before Whyte grabbed his first goal since October and his first in the league at his side's home ground this season.
Owens flicked on a long ball out of defence and Whyte ran in behind the Linfield defence, rounded the Blues stopper and flicked the ball into the net.
Forsythe caught Glendinning off his line in added time as the Blues pushed for an equaliser which may have kept any realistic hopes of maintaining a title challenge alive.
Crusaders are now unbeaten in 15 league encounters on their home patch, their only defeat of the campaign on their own ground coming against Portadown on the opening day of the season on 8 August.
Crusaders manager Stephen Baxter: "It was a pulsating game with a great cup-final type atmosphere and it was great to be a part of it.
"It is definitely not over as we have two very difficult away games against Cliftonville and Glenavon to come and we have to stay focused. We still have a big job to do.
"They are all big matches at this stage of the season but it's important that we won on our home ground and tried to drive our advantage home."
Linfield manager David Healy: "We just didn't get the rub of the green today and we missed a few chances, but it's a difficult place to come to and credit to Crusaders, they should go on and win it now.
"We needed to win this game but now we must make sure that we finish second and qualify for Europe, plus we have a cup final to look forward to."
Late goals from Gavin Whyte and Jordan Forsythe saw champions Crusaders beat Linfield 2-0 at Seaview and extend their lead over their nearest rivals at the top of the table to eight points.